This trick works that well because you applied the existing rules incorrectly.
Pulling a rope out is an Interaction with an Object. That part you did ok.
But tying a rope, however, is an Action, not an use and Object action. I mean the full tying of a knot, not merely simply pass an already pre-made loose noose to catch a hook, or something equivalently trivial.
Throwing the rope down the window is not the same as merely dropping the entire rope to the floor (which would be a Free Action). Thus, this can go from a mere "Interaction with an Object" if let's say the PC is standing adjacent to the window, or a full Action if he actually needs to "throw" some part of the rope. So, some precision is needed here! The AC to hit might be low, but it is still a "to hit ranged attack" with improvised weapon wth a short range 20 feet and long range 40 feet.
And don't let him do it as a mere Bonus Action, even if he is a Monk. Let him use his Bonus to do an unarmed strike, sure, but throwing stuff to a specific spot should always be a ranged "attack" Action. The only time it might be less than a full Action is when it is not important where and how far the item is going to land. In which case yeah a bonus action "counting a bil like an off hand attack" might work. But targeting something through an open window? It definitely should take an Action.
Grabbing, even a willing target, is also an Action.
Moving with someone you grappled: you move at only half your speed.
Just what kind of window are we talking about here? From the way you describe things, all the windows in your world are already large, tall, easily climbed through, and WIDE open or made of thin candy glass. Use normal medieval windows instead.
More like this:
"Sliding" windows, or "two panes opening to both sides" windows (thus which require twice the work to open), are also valid. Glass was someting costly! Check glass bottle cost to get an idea! Heating up a house in winter was costly too. Thus, windows tended to be on the SMALL side of things, even for middle class artisans. Only rich people had big windows. Poor people windows were tiny and often only the halflings and small kids could move through such windows.
And not like this:
Even if a window is "open", it is not necesseraly "wiiiiiiiide open". It can be opened already, but... only a little bit. So, basically, just treat as closed but not locked.
Notice how for medieval windows the glass panes are actuallly small, and fit inside wooden frames? You can't just jump through that kind of window willy nilly like those hollywood bar fight movie scenes. Treat the window as an object that it takes an ACTION to open (or destroy, using DMG Objects rules for a normal window I'd put AC 14 aka midway between glass and wood, and 3 HP).
Modern window can be opened/closed relatively fast. For medieval windows, sometimes it required a bit more work. At BEST it's yet another Interaction with an Object to open the window. But old less maintained buildings often "aged" and "worked" a bit. This means many old windows didn't necesseraly slide open or closed all that easily. In my own modern appartment, I have to grab the window frame with both hands and pull VERY HARD to the side to try to "pry" it open. so, go with the logic of the building. Rich and very well maintained? Ok! Not so rich or not so well maintained? Things might be much harder. Same for doors by the way: op0ening closing a door as an interaction with aqn objec5t assumes an easy to open or close, already unlocked door. That big 5 feet wide by 8 feet tall by 4 inches big and thick oaken "dungeon door"? Even if it ain't stuck, it is not going to be opened as easily.
And you are trying to lift somebody up 3 or 4 feet high, trying to go THROUGH these wooden frames, and allso with risks of slashing glass damage as you force your way through? Not a huge problem from the PC. But NPC damsel in distress, with only 4 HP? Ouchie.
Unless the carried person is light, crossing through a 4 feet high opening is more like a Climbing ACTION than a mere "move and jump through". An Acrobatics or Athletics check is probably needed here. If standing up from Prone takes half your movement, surely climbing onto a 4 feet high window frame is at least the same?
As per RAW, do not allow your players to downgrade (convert) their Action into a Bonus Action.
As per RAW, do not allow your players to downgrade (convert) their Bonus Action into an Interaction with An Object Action. They have to spend an Action to get a 2nd Object Interaction.
A closed window can be treated as also being locked, too. Thus, yet another object interaction just to first unlock it before opening it.
Use Climbing Rules to climb down. Failure and it's a fall and both take damage.
Maybe do not apply ALL of those aspects. They are all mostly RAW but taken together thhey add up. But still, the "out the window" trick should look a BIT action-cost prohibitive. But should not feel like an insanely dumb proposal.
But you also stated another thing: that PC (presumably he has high movement) that picks up not a "damsel in distress" NPC, but another PC instead. Presumably, this allows the "combo" where your "grabber" grabs a friend, carries it a bit, drops the friend, who then uses his own movement on his turn to reach enemies that he could not have reached at all in the first place without the grabber's help.
This limits that tactics to a small circumstantial range of enemy distances. So maybe just not worry too much about it too much?
Or maybe strongly consider using the official Encumbrance Rule Variant. Suddenly, carrying somebody else will slow you down, like, a lot! Even if DNM uses only half the carried PC's weight (including equipment!), it's still mostly carrying that other PC. Otherwise, the creature definitely must use its own move at its own initiative count, right?
Now, I still see one very cheesy tactic:
Grappler PC goes to grab a PC ally in melee with an enemy, to carry his friend out of melee range just to let that PC friend act without provoking an attack of opportunity.
By RAW this tactic can be made not to work:
Moving a Grappled Creature. When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.
And the rules aren't super clear about wether if you grapple you should have to enter the same square or not.
It is the DM who interprets the ryles, including the words "with you", not the player. You could say that ordinary grappling does not require entering the same square, but fully carrying another to move him or her means you must enter his square (trying picking up somebody from a few feet away!), and thus it means both PCs are now in the SAME square. Meaning: the grappler is the one taking the opportunity attack for the moving "away".
Also, ok to say that this kind of manoeuver is pretty involved and requires use of BOTH hands and BOTH legs and even resticts torso/head movement. You're carrying someone = no more monk unarmed strikes head butts leg kicks or whatever.
Find something that works for your group, but here you have a cheesy tactic that works because you allowed it. Following RAW is not necessarily conductive to RAI.
Your players might complain, so just talk with your players.