I think all of you who believe Ryuuji is not at fault are wrong. His mom met Taiga's dad for how long? Not even a minute, and she came to the conclusion that Taiga's father doesn't really hold other people's best interest at heart. Ryuuji was with him for an entire lunch and failed to notice this because he was blinded by his daddy issues.
Granted the meetings were under entirely different circumstances, but I think that people never really change, and if you're observant enough, you can tell what their true nature is when they slip up (i.e. Kawashima Ami).
Then again, I'm not sure why Taiga herself doesn't stop this whole mess. You can go out with someone you hate for the sake of a friend, but surely you'd stop before it completely destroys your life... Maybe it's due to Minori pushing her (assumed) parental issues onto Taiga
That would be sort of a shocker (not)