Could anyone give me an idea of how you would refer to someone who was your friend, but is no longer?
She is an ex-friend of mine? A prior friend? Something else?
It depends on the context:
If the person is no longer your friend because they are deceased, then use the term “late friend” (most people would understand that they are not merely tardy)
If the person is no longer your friend because of some kind of falling out, then the term “former friend” is appropriate, as is your suggestion of “ex-friend”.