How should I approach this? One of my female friends is being rude to my girlfriend.?

In private make K aware of the fact that you will not tolerate her disrespecting your girlfriend, and that you value her friendship and would like to continue being friends but if she doesn't stop being rude then you have no choice but to cut her loose from list of friends. Because if she were truly a friend she wouldn't do that anyway especially w/o cause but I'm thinking maybe K had deeper feelings for you that you never picked upon cause that's the only reason for a female to act that way if she doesn't have prior knowledge or relationship with your girlfriend.

Does it bother your girlfriend much? Or is it something she could live with? Sounds like K has feelings for you. You should find out what K's problem is with your girlfriend and tell her to knock it off. I know it is hard to have to choose between 2 people you care about but if one is being irrational for no apparent reason then there is not really a reason to keep them around.

K is jealous. I am not saying it is justified or not. She has been the center of your attention and now she isn't. How do you want to handle that? Will your gf be ok with including her in some of your activities? You have to weight this for yourself and decide who gets your attention and how much of it.

shes jealous you need to confront her and tell her if she wants to remain friends that she will respect your gf and treat her with rezpect

Kick her in the baby maker

tell her to F off ur GF