To be fair, no one told Geralt about Yen. Not only Triss, but Vesemir, Lambert, Eskel, Dandelion and Zoltan didn't. My guess why is because they all assumed Yen was dead given the state Geralt was found in, and didn't see the point in reminding Geralt of her.

And besides, in Witcher 2 Triss was crucial in him getting his memory back (including remembering Yen). And she was also still the first person who told him about Yen (after he asked about her after partially remembering his death, granted).

Don't get me wrong, what Triss did by romancing him when he had amnesia is still a crappy thing, but she did it because she loved him and I think her actions in W2 show she grew beyond that behavior in a positive way.

Both Triss and Yen have done crappy things, but like I said about Triss, they both grow throughout the franchise and are both good people. It's a huge credit to the writers for pulling that growth off in such a cool way.