RAW, it's valid if the ranger is strong enough: The push, drag or lift rule.
Thanks to the comments under this answer, I realized that Grappling can be used but doesn't really take into account the fact that the target is willing. Then, I thought: "The Warlock is already as fast as an anchor, what if we stop considering them as a creature and just as loot to transport?". Here comes the "Push, drag or lift rule":
You can push, drag, or lift a weight in pounds up to twice your carrying capacity (or 30 times your Strength score). While pushing or dragging weight in excess of your carrying capacity, your speed drops to 5 feet.
This allows the floating warlock to be pushed around by another character, as long as they are strong enough!
##But I think your first ruling was totally okay
Even if it wasn't RAW, rule of cool totally applies here, I think your ruling was good. This was a great idea and you rolled with it. Great job.
