How to help a friend deal with her parents?

Alright, wow, I never thought I'd have to write one of these boards.

I have a friend who is trying to go vegan, but is afraid of brining it up to her parents. she lives in an ultra-strict Christian household (The kind who think birthday parties are the 8th deadly sin.) She tried going vegetarian when she was younger, but did the typical "nothing but salad" diet until she got sick. And she learned from that that her parents will not be supportive, plus now they have ammo against her.

I have no idea what advice to give to help her. Anyone familiar with me on these boards knows that I'm not very good at being passive in my arguments, which is probably her best bet (definitely not anything resembling my arguments)

I also have no idea how people talk to their parents on controversial things such as that. I grew up in a kind of household where when I announced I was going vegan, my response was just "Make sure you get your vitamin B", not an argument, and not because I grew up neglected, but because my mom never really felt she could control my beliefs. So I have no idea how normally structured families work.

Any help?