My father once told me to never advise friends regarding their relationships (spousal, kids, friends...). He had exactly your experience in mind.
Amen (sorta). I think a good friend (where I feel that it is reciprocal) is where I dare cross the line and share honest advise. Tit for tat if you will, accountability and all that. Had two situations where I failed miserably because the guy was not necessarily a friend as he was family (of sorts). The other was a guy I grew up with and was a good friend. The second situation first: the guy I grew up with. He was cheating on his wife, was a womanizer, and treated her badly. She was a saint of a lady, a kind and gentle soul who tried the hardest to make the marriage work to my jerk of a friend. So when she called me to tell me they were splitting up, I quickly invited him to dinner and wanted to know from him what they heck he thought he was doing. He just flat out told me he loved being with other women and that it was just the way it was. I flat out told him he was an asshole and not the same guy I grew up with. He said "Yes I am, you just never took time to get to know me well enough". Here is the rub, when I got married, I invited him to our wedding and at the dinner and dancing afterwards, my new bride leaned over to me and whispered "Mario is a jerk! I think you should go talk to him to leave the women alone". I was stunned and that night with Mario at dinner remembered how my wife took one look at him and warned me he was a walking hormone. So Mario and I never spoke again, he didn't take my advise and try to work hard to save his marriage and really, what Abigail told me many years later was that Mario told her what I said about him (wanting her to have some sort of sympathy for him because he wanted to come back to her). She slammed the door on his face and said "Noe is right, too bad you didn't listen to him".
So that one went okay, even though sad to say my friend was one huge asshole that I don't have any relationship with any more. I felt bad for many years because I kept trying to think how I could miss the fact that my friend all those years was not evident to me that he was a bad guy (*towards women*). Sad to say, I was blind all those years. The other one went horribly wrong and I was sure I was going to have to do some major repair job on my credibility with my sister-in-law and mother-in-law. What happened was that my wife's brother-in-law was also a very bad hombre and one day he did some very bad things to threaten my sister-in-law, to the point that she left him that night and after that night, she decided to divorce him. My wife was very happy because she hated the guy, so did my mother-in-law. So he comes around during a weekend that we were visiting my mother-in-law and sis-in-law. He is parked outside and keeps yelling for my sis-in-law to come out and at least talk to him. My wife, my sis-in-law and my mother-in-law all look at me as if to say "are you going to do anything about this?" Damnit.
Okay, so I go outside and ask the guy to step out of the car and talk to me. He does, but he just falls apart and is one giant blubbering blob of a man who can't stop crying and is just one huge mess. So what do I say? Yeah, you guessed it "It's okay man, things will turn out okay"... I should have said "You damn well deserve your life falling apart with all your macho bullshit! Now, pull yourself together and leave and try to have a good life, okay!". Nope, stupid me tries to me Mr. Nice Guy and I spend all night listening to him cry victim in this whole thing. All I said to him was this "That's too bad, but all you can do now is try to put your life back together and walk out the type of manhood that shows you are a nice guy and one that deserves to be taken back because of it." Oops, that did not come out like I meant.
After that, all hell broke loose for several weeks. "Noe says you're wrong!" "Noe says that your Mom is to blame (what the hell, where did that come from?)" "I AM a good guy, ask Noe!" Damn. So one night, my wife, my sis-in-law and mom-in-law have me cornered on the telephone asking me to explain juse what the hell I said to this guy. I was mad at myself that I didn't say "Don't get me involved" that night, to everyone! Instead here I was stuck in the middle of someone else's mess. Why? Because they freaking don't know how to be sane, rational, good human beings with each other and they tend to drag you down to their mess every time... that's why! So all I said that night was "So, he has been saying all this stuff that supposedly I said in his defense, eh? So tell me again.... why are you leaving him" "Oh yeah, he's not a good guy... right?" "So why is he all of a sudden credible?"
I survived that night, but many years later when the other brother-in-law (of my wife) got caught cheating and was going to get kicked out of the house, I was so ready to be hands off. He invited me to Starbucks to talk to me, so I went, drank my coffee, listened while I thought of songs in my head and he sounded like "blah, blah, blah" to me. At the end of the night, it was basically... "sucks man, eh?" and I left.