People change a lot over the years, and personally I couldn’t see myself being friends with any of the people I was friends with in primary school anymore. We’re just completely different people and barely talk to each other in our final year of school now.
You could try to slowly stop contact with her. So it seems a bit more natural. Maybe give her lame advice about her boyfriend drama? The old “damn that sucks” always works for me LOL.
You could always tell her upfront as well, but I understand why that’s a bit scary for you considering her mental health. But you are completely not responsible for that, either way.