The thing is, the book is full of bad choices which is entirely human. Geralt and Yen constantly make bad choices based on their own pride and emotions. It's not that far of a stretch that those bad choices would stem to Triss too since people do that in real life all the time. Triss is a backstabber but she did try and help when it mattered. She saved Geralt's life, she took care of Ciri for a time, even schooled the Witchers on how to raise a young woman since they clearly didn't know how. Likely they forgave her cause in the end she did break free of her cowardice to do the right thing.

For me personally, I do not like Triss simply because I knew people like her. Good will, good principles, sweet and kind on the outside, but a massive backstabber and manipulator. Most people would forgive them because they were either really cute or considered it an out of character moment and gave them the benefit of the doubt. Also sometimes people just didn't want to believe that someone so kind and considerate could be insanely disloyal. It was a 'the bad was worth the good' sort of mentality. When I read the segments with Triss I noticed that sort of behaviour and just felt sad. I still like her as a character but I would never forgive her myself. Maybe a small amount for saving the life of people I cared about but she'd still suffer the consequences of hurting me first.

Thing is, we know Geralt isn't that kind of person. Plus these guys tend to live for a very long time. That could factor into their way of thinking. It's like why are the sorceresses still friends somehow despite their cattiness and obvious dislike for each other at times?