Maybe you just have a different view on relationships and friendships OP?
This: 'She does have form for this and has cut off other friends'
Would suggest you do. I do not agree that there is a rule that says that platonic friendships, all or them, or 'special' ones have to last forever / a lifetime.
Some do, and like romantic relationships that do that is a rare and precious thing.
But we all understand that romantic relationships can end, that it can be very painful for either side when they do, especially the one left, but that either party has a right to end them, and that 'I don't want to be in this relationship any more / I feel differently / I've changed and want different things' are all acceptable reasons to do so.
Why should friendship be different? There is massive pressure on female friendships to last forever.
I've ended a few friendships, either because I'd changed but the friendship hadn't, or I'd come to realise the friendship was not in my best interests.
Like early posters suggested, i had some friends who were invested in my having a hard time and especially a poor love life and undermined signs it might be getting better. I 'broke up' with them and found more romantic success afterwards.
They would say that they have no idea why, that nothing had changed, etc. Well, they missed that I was changing. And also were not self aware enough to realise how invested they were in old insecure unhappy in love me, and how they used my failures to make themselves feel better.
The flack i got from some mutual friends and acquaintences when i did this was amazing. Crazy pressure to carry on / patch it up, even when i described how it had become actively bad for me.
It's healthy to be able to end relationships that are not longer serving you.
In fact i am a bit suspicious of people who say they never have / would. Aren't they growing, changing or maturing at all?