If her best friend is a guy, it could cause problems for your relationship because there will always be another guy with whom she's emotionally intimate. If they've known each other for a long time, she'll naturally feel comfortable hugging him and more than likely has confided in him a number of times on some very personal issues. He will be there for her when you're not and can offer her insight into your behaviors, especially when the two of you are having an argument. Because of this, you need to gradually phase him out and become her "go-to guy," the first person she thinks to call whenever she has good or bad news.
As you determine whether or not her male best friend poses a threat, you need to find out more about their history. Offer to meet the guy in order to get a feel for what his true intentions are. Doing this will allow you to determine whether he's a threat to your relationship or not by doing simple things like inquiring whether or not they were ever more than just friends, watching for flirting between the two, and gaining insight into his relationship history. Gathering this information about him will allow you to deal with the situation accordingly.
Trust is key in every relationship, and the worst thing you can do is express unfounded jealousy and possessiveness because it makes you seem insecure and could drive her away for good. However, if your woman insists on always wanting to go over to her guy friend's house to hang out instead of spending quality time with you, buying him gifts, receiving mysterious phone calls from him, sending secret text messages and getting defensive whenever you mention the behaviors, there's probably more to it than just friendship and you have a reason to feel uncomfortable.
If she is really committed to you, then she should respect you enough to set some boundaries with her male friend, no matter how long she's known him. Although you can't make a woman do anything, you don't have to sit there and be a sucker. If she refuses to respect your feelings, it may be time to move on.