Depends on a few things...
How old are you? Is this a 'serious' relationship?
If you are just shagging this chick, then you don't need to care what her best friend thinks.
If this girl is your soul mate, or 'the one', then YOU should be your gf's best friend. Her friend is just going to have to realise that that is just the way things are.
This is not your problem though, this is an issue for your gf to deal with. If her friend is feeling neglected, then she can stop being such a child and try to make more of an effort to catch up with your gf. Either way, spend as much time as you want to with your gf and you only need to be concerned if you think she is starting to neglect you.