Tchia was nice enough when I tried it recently. Haven't played too much of it though. You can become the pet by soul-transforming into animals to solve puzzles!

Sea of Thieves, once you get around slooping solo and being aware that PvP is avoidable by a good captain - but it takes time to become a good solo captain. You can buy a pet I think. Not sure if that's for real money though. You can buy a pet rock.

EDIT: Just for the trololz - how about she tries Elden Ring? I hear it's a wonderfully relaxing, easy game. You can summon your own goat-horse and also other animals (like wolves! They're fun). Whenever you die - if you die, I mean, it's a wonderfully relaxing, easy game after all - you respawn and can try again! Combat couldn't be simpler, it's attacking, dodging, and blocking. You can avoid PvP by playing it entirely offline too!