just dont get played thats all. youre aware that he hangs out with other chicks so dont try to hide that you know. i mean you shouldnt go accuse him cuz you arent together. but i think being honest is the best bet. you could talk to him and say.. "hey my friend saw you at that resturant... do you go out often? is it a cool spot? maybe we could hang there"... or just sorta lead the converstaion to find out if he would be interested in getting to know you and all that good stuff.
im always honest like that but maybe thats why i am single.. lol.