LF- Lavender knows who the guy is, and you can’t assume that she is not being “mekabel” at all. The whole danger of LH and Motzi Shem Ra is that people are very affected by EVERYTHING they hear. Many people in her situation would start to have a negative attitude towards the boy and it would affect how they think about him, even if only a minuscule level.
In regards to your saying that she knows that what is being said is not true, that would only be the case if we were talking about something clear-cut. If someone said that he has purple hair, you could say it’s not Motzi Shem Ra because she KNOWS it’s not true. However, we are talking about something that she DOES NOT KNOW one way or another. It might be that he has commitment issues or it might be something else.
She has a chiyuv to try to assume that his breaking up with her does NOT reflect on him negatively (to the extent that she can do so w/o feeling bad about herself), and we are not allowed to say things that will make it harder for her to do and put negative ideas about him in her head.