Most Helpful Girl
A good way to find out is to ask if you can join them. If he seems uncomfortable and comes up with excuses not to, then something is up. If they are truly just friends, then the more the marrier.
Warning signs to look for is if she is too eager to be YOUR friend and compliments you and butters you up a lot. She could be trying to gain your trust so to throw you off her scent so u don't suspect anything. She should be getting to know you, not try to be your best friend. I had an ex that had a girl buddy. Every time she saw me she did the whole "hiiiiiiiiiiii! " and gave me hugs and was overly chummy with me. One night we were all hanging out at his house, and I turned on the voice recorder on my phone and set it out of eye view. The n said that I have to run to my house real quick to get something. When I listened to it later, I heard their conversation while I was gone. They talked about whether or not to have a quickie while I was gone. Oh, be sure to turn your Ringer off first. But only as last resort.