Should i leave it be?

my friend's boyfriend is beating her. i sometimes see bruises and a little blood on her arm and from her pants (yea when i saw blood coming down her leg, i look up her skirt...i asked first xP). she kept saying she fell or had a period (to explain the bleeding). i'm a guy, and don't know much about periods, but she had a cut right NEXT to her about a few inches away. she doesn't want me to do anything about it, and she keep saying she loves him but...i love her enough to ignore all that and do something anyway xD. what should i do about her boyfriend? or should i do anything at all? (to be fair since he cut my friend, i'm going to be using a switchblade ^_^)