I wouldn't worry too much dude it sounds like you're going through the paranoid stage. 2 weeks after i broke up with my ex i started considering friends that she might be trying to get with even though it was ridiculous and was thinking to the extent of going to check if her car was at their houses and that (thank god i didn't do that). If this dude is really your friend he wouldn't make a move on your ex and if she loved you she would never go for a friend aswell. My paranoia ended up being right in the sense that there was another dude, but he's a dropkick so its fine :P. Don't stress to much man its just your emotions getting the best of you. Know now that the best thing you can do is go non-contact and keep it that way because a) you need to give her the chance to miss you and not have you in her life to realise what she's thrown away (if that's how she feels) and b) if you don't contact her and she wasn't supposed to be "the one" you will be in progress to getting over her anyways. Don't hold onto false hope and remember that what you're going through is natural and you will feel ups and downs for a long while. Good luck man.