How do you deal with people who try and run your life?
Regarding other personal issues - same thing. I tell only a few people about any major decisions I'm contemplating. Over the years I've learned who has my best interests at heart and who doesn't. Sometimes people who are close to us really do know what they're talking about and they do give the best advice. Sometimes, however, it's difficult for them to be objective. Sometimes it's hard for us to be objective about the decisions we make - whether it's about dating, work or anything else. If you do talk to your friend - listen to her advice with an open mind. She may say some things that you don't want to hear - but she might have some valid input.
Re. M - it's kind of early though. Perhaps you should wait and see if you actually meet each other and if you end up actually dating before you start talking about him to your friend.
P.S. (I hope my post is coherent - I'm not quite alert yet this early in the morning ;-)
If you go out w/M a few times and think there might be something there, then if you want to tell this friend of yours about him, be prepared for her reaction and keep in mind that whatever issues she has w/M's situation, the important thing is that YOU are fine w/him. So if she starts saying negative things, just say "I'm sorry you feel that way" or something like that, then change the subject. Good luck! :-)
The reason why I'm worried about not telling her is because she always tells me about guys she's met online.
To refuse to associate - that'll relieve you of hearing her opinion. It won't stop her from having it and voicing it to others.
What she thinks is fine to know....it's something to be heard objectively, considered rationally, and assessed factually. But you're not required to respond to it with words or actions....except "thanks for your opinion, I'll take it under consideration" and even then, you're only required to respond like that if you're wanting to be polite.