I think that given the fact that your woman has stated that he isn't very smart and he's arrogant is a big clue into just how NOT into him she is. Women and men are often on different wavelengths when it comes to attraction. Guys tend to think more strictly in terms of physical attraction (as you are doing now, which is why you feel jealous), but women are much more attracted to the mind as well (which is why your woman is not into her guy friend at all). And I'm not trying to stereotype, this is just how I see things. You probably see him as an attractive male, but speaking as a female, when a guy is dumb as a box of rocks, he suddenly looses his physical appeal and isn't attractive in the least.

I also think it would be a good idea for you to work on your own insecurity issues. Your woman has a right to have male friends. If you become jealous and make things bad between her and her friend, that's really not good. (It can be seen as abusive to isolate a partner from his/her friends.)