Boyfriend and Best friend..., Are having some probs... Options

[ZukoJin]

My boyfriend and best friend sometimes get in these fights sometime. They start off sorta play fighting but sometimes they get more serious and someone gets hurt. Like this last (and worst) time she kept saying he owed her some money (even though he didn't) and then stole his wallet and took the money out then threw it at him but while they were fighting over the wallet this glass orb thing fell off the top of her wardrobe thing and hit him on the head and all and she felt real bad and all but that day they seemed to get over it and be okay again. But then the next day I was on the phone with him and he mentioned that his head still hurt so I told her on IM only I made it sound a bit worse so she'd feel guilty and start being nicer to him. But instead she goes and gets all depressed and now I don't know what to do. I feel like it's my fault because I said something to her and all. I've told her I'm sorry and that my only intention was to get them to get along all the time and all but it didn't seem to help. He's pretty much over it now but she's still beating herself up about it and I feel responsible. Other things she's done is use him as her personal stress ball (he still has scars from where she dug her finger nails into his arm), kicked him in the balls several times (mostly accidents I think...or so she said) and hits him on the head all the time. Now him and my sister fight a lot too but it's all plainly play fighting and my little sister is 12 and a lot smaller (she's a skinny little toothpick) than my best friend who is 15. Don't get me wrong my boyfriend and best friend get a long great it's just sometimes they're "play fighting" gets out of hand and I really wish it'd stop. How can I get them to stop without making anyone feel horrible about themselves?