I'm probably not the best person for relationship advice (like... at all) but it seems to me that since both physical and legal action could create problems between you and your GF, you're best option is to fill her in completely(as others have said) as well as explaining that it's bothering you and ask her how she's ok with you handling it or, if she doesn't want you to handle it, ask her to. If she doesn't want you to handle it and won't handle it herself, I'm just gonna say it's probably not the best relationship.
Don't really understand why you wouldn't tell her everything anyways, I always thought that being completely honest and open with your partner was best, to eliminate problems, but like I said, I'm probably not the best person for advice on the subject.
[Edited on 01.06.2013 9:18 PM PST]