I have a player running the revised BM ranger, and it's pretty ridiculous. Easily pumping out 30-50 damage per round, on average, at level 6. 3 total attacks, 2 from the animal with boosted stats, plus a mark and *usually* sharpshooter. He's making the GWF Champion look pathetic. In another 5 levels, that animal is going to be doing an AoE attack against everything in reach...
30 to 50? Something seems off in your calculations. What animal and weapon is s/he using?
The animal honestly feels like another player character, with all the stat boosts and overall combat effectiveness it has. There are definitely some balance issues, but we're too late to change that unless I manage to kill the character.
You are running something very differently than the way the game is being run at the table I am currently playing at. I'm at level 7 right now. My wolf has only 35 hit points with an AC of 16. It goes down, and it goes down fast. It is easy for the DM to put an end to my animal damage boost, at which point in time my damage usually drops as well (because instead of attacking I scramble to try and heal my animal companion before it fails 3 death saves). I've been keeping fairly careful track of my overall damage per encounter, and while it tends to spike high, the mechanics of the animal companion also make it drop low. All in all, keeping a very careful eye on comparative numbers between my entire group (over the course of encounters and not just turns), there is definitly a problem, but the problem isn't as bad as you are making it out to be (at my table).
If I might make a suggestion: speak to the player about making a small modification to the sheet. I am the player in my group, and I just asked the group for permision to remove the proficiency bonus from my animal companion's damage rolls as a house rule. Speak to your player about making a similar change to his sheet and see if that makes the game more enjoyable. Also, if you are not attacking his animal for some reason, start. If he runs the animal into a tactically dangerous space with lots of monsters around, have them gang up on it and knock it out. Force him to position his animal on the outskirts of combat if he wants to keep it protected. When it goes down, use its death saves as a ticking clock that forces him to spend his actions and spell resources on reaching and then healing it. Your game might start to feel more balanced.
In any case, this is the spell I proposed to the group:
Magic Fangs
2nd-level transmutation
Casting Time: 1 bonus action
Range: touch
Components: V, S
Duration: Concentration, up to 1 hour
You touch an animal connected to you by the Ranger’s Animal Companion class feature. Until the spell ends, its attacks are considered magical with a +1 bonus to attack rolls and damage rolls.
At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the bonus increases to +2. When you cast this spell using a spell slot of 5th level or higher, you do not need to concentrate on the spell to maintain its effect.