It would definitely raise a red flag with me. But I wouldn't go as far as to say she couldn't have guy friends.
As I've developed my skills I've started learning who is a real threat and who isn't. There's some guys who are just so beta or so lame and you know they have absolutely no chance in hell. However, you do occasionally run into a guy that can spit a little game. The best tactic is to always act like your better. But don't just act. BE the better man. Make her think that if she is with anyone other than you then she is stupid. Make her think she would be absolutely stupid to ever fvck up. When I see a threat I take affirmative action. When you make this other "player" feel belittled, he will often retaliate and talking some smack. When people talk smack, consider it a good thing. There's nothing better than an ego boost then people constantly having your name in their mouth, or constantly reminding a girl why she shouldn't be with you.. it just makes them want you even more. At the end of the day I think it's all about confidence and how you handle things. However, if a girl has a lot of "guy friends" I usually consider that a red flag. And that would be a girl I wouldn't get into a relationship with in the first place. If something doesn't seem right, take action. The longer you wait, the more she thinks she can get away with that sh!t. Playfully let her know that guy friends are okay, but you shouldn't be talking to them excessively, or hanging out with them a lone. A good girl knows better than that. Just use your judgment, and trust in hers. If you ever feel like something is wrong, then there probably is. Next.