I'd have her do some research on it. Find out some recipes, try them out, find out what substitutions can be made. Protein isn't really a problem- it's very hard to get a protein deficiency if you are eating enough calories of decent food (even if just grains, veggies, fruits, etc)
http://www.bing.com/videos/search?q=...0D17&FORM=VIRE
This movie is funny but the research is there- she should be fine even just leaving out the eggs/dairy out of stuff.
I think you should tell her if she wants to do this, she'll have to do most of the extra work herself. But it's not hard to make a tiny pan of lasagna with no cheese.