Hey Pixie, thanks so much for reaching out! It's really kind of you to think of your friend and how to help them.
As such, The Mending Circle is meant as a playful way to explore some of your own issues through the lens of other characters. However it's not a tool that can replace therapy or anything like that. In the end, I think the question is whether your friend would be comfortable exploring their issue through play. The game does it's best to give all players control how much they want to delve into things but it's still a very personal question. In any case I definitely recommend playing with some kind of Safety Tool. Either the one supplied with the book or something else.
I hope this helps. If you got any more questions, feel free to ask!
Oh, and thank you for your kind words about Ex Novo and Ex Umbra!