When you open a loyalty tome it gives you the option to give 10 levels + 25 of each other stat to one of your pets.

I have a capped pet but it still gives the option to use on him. Will it give him the stats or just disappear, anyone know?

The most likely explanation is that because levels/int do not count toward the cap, your pet can still use the loyalty tome for that.

However, because it is also supposed to boost the stats, I cannot say if that would be affected or not. When I was boosting my own T13 with regular tomes last year, he went over the cap temporarily but I believe once the site registered he was at 35k+ it stopped giving anything other than level and int. I wouldn't recommend experimenting if you have another pet that could benefit from the +25 to all other stats.

you could also make a ticket about it. surely the staff knows ;)