Would you be saying the same if the parent decided that the kid was forced on any other diet that isn't vegan? The kid clearly wants to eat meat, and eating meat is nowhere near as bad as alcoholism... There's also probably a reason why the kid doesn't tell his mom, if she's one of those near-religious type of vegans then the kid knows he's gonna have a bad time if she finds out. The best option is for the aunt to talk to the kid and try to figure out if he wants to tell his mom and why he hasn't so far, I wouldn't out the kid if he believes he's gonna be treated differently for it, just like I wouldn't out an atheist / gay kid to a deeply religious family.