Once Yennefer exorted Geralt to settle with her for some time in Gynvael (located in the far north) to pay a visit to a sorcerer, "friend" of her, because she had to do some "work" with him. Geralt is pissed off, because Yen always quotes and talks about him, and eventually discovers she is having an affair with the sorcerer, whos name is Istredd. Geralt and Istredd have an encounter and resolve to have a duel for her, despite both knowing that Yennefer would be angered with the one that survives (and that means danger).
The story goes on and Geralt resolves not to duel, after some events I can't recall too well and receiving a black kestrel that Yen had made with magic, which had a letter, explaining that she couldn't make a choice, so she was leaving both of them.
So, in that case, Yennefer was the one who broke up with Geralt, and the poor witcher was again alone in the path, but having learned a lesson.