I don't have a cat and I don't know much about what you feed them. I do have vegan friends with dogs and some of those dogs eat a vegetarian diet. I know that they supplement their dog's diets. I'm not sure that is possible to do with cats. cats (unlike dogs) are obligate carnivores.
A while back there was a thread on vegan cats.
https://veganforum.org/threads/im-a-vegan-and-sos-me-cat-bbc-film.2826/
I don't think there is any useful info in it. but I would recommend that you have your vegan friend discuss this with her veterinarian before she tries anything. (My local vet sells vegetarian dog food but as far as I know there are no vegetarian cat foods.) the above thread references a facebook group of vegan cat owners.
I've seen cats play with bugs and eat avocados. So maybe an avocado and bug Friskee dinner is possible.