If someone’s inappropriate actions caused others to go off the derech, can he do Tshuva, even if he can’t get them to come back? (Meaning- the person who went off the derech is now happy with his new life and wants to stay the way he is now.)

The gates of Tshuva are always open for anyone who sincerely regrets and wants to change. It’s true that many actions in life cannot be erased retroactively, yet we are told that this is exactly the miracle and greatness of t’shuva. Hashem created us, and knows that we make mistakes and aren’t perfect, so He also created this solution. It’s never too late for your friend either, so if you can bring him back, that’s even better!