Should I tell his new girlfriend?

I went out with a guy for several months. We lost touch and both began seeing other people. I ended my new relationship and my old ex and I began talking again. In that conversation, he told me he had a new girlfriend, yet he wanted to see me anyway. I said no because of the new girlfriend, but something bothers me now. Should I tell his new girlfriend that he wanted to see me? Or should I let sleeping dogs lie and let her find out what a sbag he is on her own? I am not a revenge seeker, we parted on good terms and his "proposition" came as a shock to me. I thought he was a good person and I feel bad for her now. So, should I send her a discreet message? Or should I let it go and wish her the best of luck?