im a very social person, and have a lot of male friends, i also go out every weekend and meet new friends of both sexes. my current boyfriend has no problem with this as he has a whole heap of female friends. i trust him, he trusts me. he doesnt mind if i go out with friends even if im the only female because he knows im always his at the end of the day (not that id cheat-thats not how i mean it).
with my ex however, we were together for two years, and one reason we broke up in the end was i had no social life. he never trusted me going out. i couldnt even go out with my girlfriends without him worrying that i was secretly meeting guys etc. i never gave him any reason not to trust me, he was just really insecure and jealous. because i loved him, and he said he didnt want me going out, i said ok. and maybe this is what your friend is doing. however she does need to break out and tell him that she needs to have friends aswell (both male and female friends), because they are a support network etc. everyone needs friends and our boyfriends and girlfriends need to see that or there is no future in the relationship.