Here's the situation:
My Four Elements monk character, seeing this, decides to use the Water Whip on the ally. He's going to take some damage, but since he's restrained, he will have disadvantage on, and likely fail the Dexterity saving throw and I will be able to retrieve him.
You can spend 2 ki points as an action to create a whip of water that shoves and pulls a creature to unbalance it. A creature that you can see that is within 30 feet of you must make a Dexterity saving throw. On a failed save, the creature takes 3d10 bludgeoning damage, plus an extra 1d10 bludgeoning damage for each additional ki point you spend, and you can either knock it prone or pull it up to 25 feet closer to you. On a successful save, the creature takes half as much damage, and you don’t pull it or knock it prone.
Since this situation is somewhat different, my DM ruled that there would be a "tug of war" that took place instead of the typical Dexterity saving throw. The NPC dragging the character would instead make a Strength saving throw against my Ki Save DC. Essentially turning my Water Whip into a lasso.
His logic was that you couldn't use Water Whip on a creature restrained, and chained, to a wall and expect it to work.
So, essentially my question is:
Is it reasonable to make Water Whip a tug of war, requiring a Strength saving throw against my character's Ki Save DC, in situations where a target is being restrained?