They have been friends for so many years, he has always tried to ask her out but she says no, i'm 100% sure she doesnt see him in that way.
On his birthday party i kissed her in front of him and that night his dad called my girls dad says that he was heart broken and crying becuase he is hurt seeing her with me.
I dont know what way i can tell her without looking like i dont trust her to be honest.
Gee, who's the problem here, your GF or this guy? If you are secure in your relationship with her, tell her he needs to be more mature about HIS relationship with her. Pining over a girl that someone else has been going out with for 8 months? It sounds to me like she wants some time to herself occasionally (which is fine) but she can't say NO to him because she feels guilty about their friendship/relationship. She shouldn't have to.
Edit: and be honest (to a point). Tell her it isn't that you don't trust her, it's that you feel like it's hurting your relationship with her and you would prefer that didn't happen, because she is an amazing woman. (yes, use the word "woman" because you want her to behave more maturely). And make sure you tell her that it's not your intention to hurt her friend, but that it's having a serious impact and you'd like to work it out.
Barring all that...she may be a great person, but even great people have to learn to say NO to their friends sometimes. Who is the priority here? You or him?