With the last patch, the Blizz UI is reporting that your party members' pet's maximum health values are 100. So you'll see a pet with 6000 health at 6000/100 or 6000% health. Yes it's ridiculous, and it's only broken when the pet is close-by. Get far enough away and it reports correctly. This is making instance pet-tank healing very difficult..so I wrote an addon to address the issue.
Just download (and have your party members download) and install this like any other mod, then quit and restart WoW. It will do a pretty good job of auto-guessing a party member's pet's max health based on the current health value, which always reports correctly, along with the max health value provided that you're far enough away from the pet. You can reset this value by either walking far enough away from the pet so that the max health is reported correctly (out of healing range, I believe) or just doing this:
1) wait for the pet to be fully buffed and at 100% health
2) target the pet
3) type /pph
Please note that if the pet has 6500 HP buffed, then dies and loses all its buffs putting it back at 6000 HP, the ui will show it as 92% even though it's at full health. Fix this by either buffing the pet back to where it was before, or following the steps above to reset the max value.