I could be wrong, but didn't she know Istredd even before Geralt? Istredd also tells Geralt that she always came back to him, because her "new love" didn't work out the way she wanted, but as we know with Geralt it was another thing, because in the end she chose love over wealth.
That seducing thing was (probably) to escape them and didn't work so well as far as we know. If you have the tools, you gotta use them

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I'm wonder how they gonna set it up, if Yen get to learn what happened between those two. She wasn't really fond of their brief relationship and she threatened her to land a finger on him again, so..
And don't forget Geralt lied to Yen about his relationship with Fringilla

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