About using guilt as a tool for making them doing the right thing.
If you do it too much then the person will start associating you to feeling guilty whether it is consiously or not. No one likes feeling guilty and chances are that she will distance herself from you. Im not a psycologist but im basing it on what i would probably do if someone tried dointg that to me.
As for helping this girl. The best thing is just to be a good freind and someone she can tgrust. NEVER go behind her back and do something you said you wouldnt or tell someone something when she trusted you to tell nobody.
Also make sure to give a lot of honest smiles even a small one can help. No idea how but it will always put a person in a better mood. Maybe not right away but it may be a few minutes later when you are no longer there and she starts thinking more positivly but giving a person an honest smile will ALWAYS make them feel somewhat better.