In a lot of situations in life, I think it's helpful to assume other people are clueless and meant no harm.
With new construction, some people mistakenly assume they don't need an agent. They don't get that while the issues are often different than wish existing homes, there is just as much negotiation. For all you know, maybe your friend didn't even realize you could and should have been involved.
This will happen to you again. You might be stellar at your job but it's a crowded low-barrier industry and most people know more than one real estate agent. The average person doesn't know how to vet "Ok" from "Great". Aside from the aforementioned privacy issues of discussing personal finances with a friend, sometimes you just lose out because your friend's spouse's cousin just got their license and they want to help her out.
I don't think you should be overly helpful to those who don't use your services but want free advice but you should try to balance that with maintaining a good relationship because when your friend's cousin fails like the other 90-some percent of new agents, your friend's refferals will have to go to someone.