My brother was a vegetarian for a while during college...we'd make Tofurkey for him on Thanksgiving, etc.
Then he joined the Navy. That cured him of his silly vegetarianism
It's not a big deal. One of the women I work with is a vegan. She's got no problem with what other people eat, if they hunt, etc. She just doesn't like meat, and won't touch any food made from an animal (including milk, butter, etc..in general.)
Tho, sometimes, I bring her some biscuits from Popeye's...she can't say no to those
Contrary to popular belief, most vegans that I know of aren't beating the PETA drum; they just made a lifestyle choice about what they'll eat. They don't want to get grief over it, so why should they comment on what a carnivore eats, too?
I wouldn't worry about it too much. If your mom invited her over, she already knows what she's getting into. Your mom would have told her. Fret not