Quasits are not going to be as helpful as your warlock thinks they are

Reading from the SRD, here this is the first sentence about Quasits:

Although quasits thirst for victory and power as other demons do, they are cowards at heart.

When you combine this with the description that you posted above:

At any time and for any reason, the quasit can end its service as a familiar, ending the telepathic bond.

You get the full picture.

Yes, if your Warlock has somehow managed to get an always chaotic-evil cowardly demon to serve him to the death, then sure, it's pretty overpowered. However, as a DM you should probably not allow those circumstances to ever occur. A quasit will work with your warlock as long as it suits them, but the minute the warlock asks them to go into danger the quasit will just leave. Or say he'll do it and leave. Or say he'll do it and immediately switch over to the enemy's side and give away the warlock's plans.

In reality, the quasit will probably be more of a hinderance than a benefit to the party. At a minimum, your warlock certainly shouldn't be able to command the quasit to do anything other than what the quasit wants to do naturally (and being chaotic evil, it's probably not what the warlock wants).

Quasits are not going to be as helpful as your warlock thinks they are

Reading from the SRD, here this is the first sentence about Quasits:

Although quasits thirst for victory and power as other demons do, they are cowards at heart.

When you combine this with the description that you posted above:

At any time and for any reason, the quasit can end its service as a familiar, ending the telepathic bond.

And finally look at their alignment:

Alignment: Always chaotic evil

You get the full picture.

Yes, if your Warlock has somehow managed to get an always chaotic-evil cowardly demon to serve him to the death, then sure, it's pretty overpowered for the reasons you mentioned. However, as a DM you should probably not allow those circumstances to ever occur. A quasit will work with your warlock as long as it suits the quasit, but the minute the warlock asks them to go into danger, or asks them to do something even slightly selfless, or even as soon as the quasit gets bored, the quasit will just leave. Or say he'll do whatever he was asked to do and then leave. Or say he'll do it and immediately switch over to the enemy's side and give away the warlock's plans. Or do what was asked, wait until the party is asleep, use their invisibility to steal all of the party's magical items and then leave. Or wait until the party gets some loot, polymorph into a giant ape, punch the warlock in the face, steal the loot, and then leave.

In reality, the quasit will probably be more of a hinderance than a benefit to the party. At a minimum, your warlock certainly shouldn't be able to command the quasit to do anything other than what the quasit wants to do naturally (and being chaotic evil, it's probably not what the warlock wants). More likely, the quasit will scheme with and against the warlock at all times, and will probably immediately recognize its potential power (being the eyes and ears of the party on scouting missions). As a DM, you now get to RP an NPC who can be used to bring down the party, or at the very least be a thorn in your warlock's side until they change their familiar back to something more normal.