Mdthbs

Context

A warlock in the game that I run suffers from two levels of exhaustion:

Level 1: Disadvantage on ability checks

Level 2: Speed halved

This warlock knows the spell levitate. He is so tired—it's a hangover from a very potent homebrew drug, named Flashberries—that he levitates himself slightly to alleviate his body by hovering instead of walking. Yesterday, instead of walking/floating around by himself, he held onto the shoulders of his ranger ally and let the ranger "pull" him forward (as the ranger walked at normal speed).

Ruling

When the players came up with this idea, I ruled that this method of movement is possible. Because the warlock is afloat, the spell doesn't burden the ranger with extra carrying capacity while the warlock is not using his own movement, so both the ranger and warlock could now move at normal speed. The ranger effectively pulls forward a seemingly weightless person.

I thought it was quite the clever plan to avoid this part of his exhaustion, and I didn't want to slow down gameplay by looking up the rules, so I ruled in favor of the party.

Research & Question

Now that I'm reading up on both the spell description and What form of movement is granted by Levitate cast on yourself?, I realize we might've misunderstood how levitate works. Zooming in on the relevant parts (emphasis mine):

The target can move only by pushing or pulling against a fixed object or surface within reach (such as a wall or a ceiling), which allows it to move as if it were climbing.

and

If you are the target, you can move up or down as part of your move. 

So this makes me think that the answer to my question would be No, because the ranger is not a fixed object or surface. And since spells only do what they say they do, I'm more inclined to think that, by RAW, the ranger is unable to move the warlock at all. It also looks like the warlock is also not able to actually hover horizontally by using this spell (oops).

Could anyone give a clear yes or no to my question, by RAW? And if either of my interpretations are incorrect, please explain why that is.

You already have the proper solution

A spell's text details the spell's effects — the only thing the spell does. Any additional effects are up to the DM. The DM role is crucial, because rules do not cover all the corner cases. This is a corner case, so you can not figure out the "correct" answer based on rules only, without the DM.

I thought it was quite the clever plan to avoid this part of his exhaustion, and I didn't want to slow down gameplay by looking up the rules, so I ruled in favor of the party.

Good for you! Your player came with a creative solution and was rewarded. Unless he tries to use this movement method in combat, in terms of "balance" it really does not matter.

Jeremy Crawford, the lead game designer, suggests to choose what is "best for your story":

"The rules are intentionally silent on these corner cases, leaving adjudication to DMs. As always, I say go with what's best for your story."

That is exactly what you've already done.

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.

You're looking at the wrong thing. The warlock can only move himself in the stated ways, that says nothing about whether someone else can push/pull him along. Since he is neither resisting nor even has any way to meaningfully resist this should be automatic subject to strength limits.

I would have no problem with permitting this. Nahyn Oklauq's answer brings up the issue of strength, but the warlock is floating, not being dragged - there's no friction. If the ranger gets overloaded, I would require him to accelerate at 5 feet per round until reaching the desired movement rate, and take the same time to slow the warlock if they decide to stop. (The warlock might end up banging into something painfully!)

Note that if he's too much for one character to haul around, you could use ropes and have more than one person pull him.