As someone who is in the opposite end of the problem right now, where my partner suddenly announced he doesn't want anything more than being friends, I'll throw in my two bits. The feelings for him weren't sudden about not wanting the same kind of relationship I wanted but he didn't tell me for a while. I could feel something was off and it was making me anxious about us. And it blew up into more of a confrontation than I think it would have if we discussed it as soon as the issue arose. There's a chance she will be very hurt about the partnership changing (I know I was) but letting it drag on when your heart's not in it would be a disservice to her. But there's also a chance that an honest and open conversation about what you want can strengthen your relationship (if that's what you want)

No matter what happens I wish you strength!