ds(19) is a bit older than your dd, split from is gf(19) of 18 months earlier this year - it just fizzled out for him, no one else involved and no one new on the scene yet. She still messages him occasionally and he responds.
He bumped into her on Christmas eve when he was out with friends at a local bar and she had a very teary/intense conversation with him and has been messaging him ever since saying she will hurt/cut herself if he doesn't meet up with her (usually in the middle of the night) to talk about why they split. I have no idea if she is trying to manipulating him or genuinely upset, probably a bit of both.
It is so hard watching it happen - thinking about both of them upset, ds has been in tears over it daily too. All you can do is be there to listen, try your best to guide them through it whether they are 16 or 19. I am trying to get ds to tell her to speak to her family/friends - hand over responsibility by him letting her friends know she appears to be struggling - and then block everywhere, which feels harsh, but I am finding it hard to judge if him replying is helping her or just dragging it out/giving her the wrong signals.
It was all so much easier when I was younger as we could just cut contact and work on getting over it. The only way to connect would have been the landline or knocking on parents front doors! Although I do remember once late at night an ex climbing in the garden and throwing mud at my window to get my attention, dad chased him and on inspecting the previously white house the next morning found he wasn't a very good aim!
Hope she starts to feel better soon, can you keep her busy, take her out, so she doesn't wallow in it too much?