Given that you can't stop her, i'd go with supporting her choice and finding out how to cook wholesome vegan meals.
If you are a halfway decent cook it's really not difficult - there are loads of blogs etc of "normal" mums, families etc who are vegan and cook normal food!
Vegan lasagna - "meat" portion made with tinned green lentils, tinned toms, herbs, onion, garlic and chopped veg served with potato wedges, salad and garlic bread is a very filling meal - and can be enjoyed by all. Tesco do loads of "free from" soy substitutes - the smoked vegan "cheese" is excellent to top pastas / lasagna etc with and can be grated or sliced. Tesco value garlic baguette is vegan.
Curry is a good one too - potato, sweet potato and broccoli in a nice coconut milk based sauce with rice and naan/poppadoms.
Chinese with loads of veg and some Cauldron marinated tofu in a spicy sauce with rice and spring rolls.
Linda McCartney sausage rolls are very tasty too with mash and beans.
It's a real fallacy that vegan diets = very restricted calories and super healthy!
Any way of eating - be that vegan, vegetarian, pescetarian or omnivore can be restrictive or healthy or fattening or balanced - it's purely down to what that person wants to get out of it and how much effort they will put in to it.