Just as an example:
Same situation came up with me and my friend and another girl. They were NOT boyfriend/girlfriend they had just gone on like 5 dates or whatever and ended up sleeping together once or twice. Later on when they were just hanging out he asked her if she wanted to go back to his place. She declined because the last times she slept together she was pretty drunk. Anyways, he introduced her to me once, and then later we went on a double date me my friend, and the ex and her girlfriend. So anyways things kidna ended wtih the two of them. I asked if it would be ok if I called her. He said it was fine. He specifically said that if I were to go out with her everyday he wouldn't care. So we started dating and eventually became really close. I never lied to my friend and I thought everything was cool since he said it was fine. HE even was the one that gave me her number.. Anyways one day he asked me if we had sex and when i said yes worlds collided. It took him a LONG time before he would even speak to me again. He interpreted the situation as me "stealing" this girl from him. Even though the night I asked for her number he said "I'm never ever going to see her again." So you better be careful even though your friend might say it's coo it might not be. Probably a better idea to get your own girl.