thats easy first check his relationship status on facebook!
Just being there and listening as a good friend is about all you can do. If your best friend really needs to know about the suspicions he has about his girlfriend then he should perhaps follow her to find out what she is up too so he will know for sure because he could be worrying over nothing.
I have never actually done it, but maybe u could ask him to hang out with a group of people sometime (so it is not just you and him, so he doesn't get suspicious) usually guys are less suspect of girls when they have a girlfriend because they are 'taken'. Friend him on facebook, but don't be stalkerish :)
If you and your friend are "close" friends you have to respect the boundaries and not like the other's girlfriend..there's plenty of girls out there without having to ruin a good friendship
You practice. I started with my E minor sounding crappy and now i play most of the notes like a pro and sometimes i even jam out with my friend Cody Simpson
You can have friends that are girls without having to enter the dating scene. These are common acquaintances of yours in school or classes ... in this case, it is just a 'friend' and not your 'girlfriend'.
just talk to her. get to know her better :)
does your friend's girlfriend have feelings for you too? and you need to figure out which is more important to you, your friend and your current girlfriend? or your friend's girlfriend... i can't help you that much sorry :(
"Hey, let's go do [insert whatever activity here]". if you really have to think long and hard about this, you might want to question just how much of a friend they are.
Just talk to them like they're any other friend. Talk to them about the stuff you would talk to your other friends about? You said you "used to like" this person, so it shouldn't be too uncomfortable.
Don't be jealous, she is only his friend. After all: You can meet boys and be friends with them without being their girlfriend.
This is possible if you are not around your best friend when you talk to her or are in another city, town, state, country, or continent away from best friend.
show them your boobies.