My gf's best friend keeps comin onto me/wants to hav with me etc.?

You'd better tell her before she hears about/walks in on one of these incidents and misunderstands it completely. Then let the two of them work it out. Her friend s*** all over the friendship when she started hitting on you anyway, so she must not care about it that much. The truth in these situations is always best. And as for your relationship, your girlfriend has no reason to be mad at you if you really haven't done anything. Just tell her exactly what you put on here.