What are Friends for????

So my best friend who i will can Jane, she's a twenty something payroll accountant. She's a beautiful woman, we have been friends for ever and i mean forever since kindergarten. she's been with me through sickness, bad breakups and more. Not to mention that i have also been there for her as well, she is having a really tough time finding a man who is worthy of her, she is what we call the 2ND fiddle, no man realizes that she is for them until they cheat on her and she finds out Or they are still "friends" with an ex. What is up with that? why do men find it necessary to be friends with someone they have slept with or anything else for that matter. Why do woman think that it's OK to do that ladies don't remain friends with a man who you have either slept with or have a long relationship with. IT DOES NOT WORK. I tried before I was married didn't work out for either of us.
I can't even count on two hands how many times this has happened to her. I feel really bad for her every time it seems to look like a guy is great he's a total loser. Just like the rest. My husband and i are starting to think that we are bad judges of character.
Can anyone give me any ideas on what to do in some situations like these, i don't care what you have to say give me some advice on how to help her(I KNOW IT'S NOT REALLY ANY OF MY BUSINESS, BUT SHE'S MY FRIEND AND I WANT TO SEE HER HAPPY) She is actually on the verge of swearing off marriage and children since she is having a hard time finding the one.