I don't understand why you wouldn't care about who he dates. I thought most of us like to see our friends with a partner who treats them kindly and with love and respect. Are you saying that you wouldn't care if he got treated badly?
Likewise, it's certainly not uncommon for someone to ask a friend for their opinion on a new boyfriend/girlfriend. Especially those who've previously missed warning signs and gotten burned. Those people know that friends can see flaws which are still invisible to us. He trusts your judgement and wants your opinion.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.