The best thing you can do for her is to drag her to the doctor or planned parenthood and get her on a RELIABLE form of birth control. Today if you can. Do not wait. Do NOT get her on the pill, do not rely on any teenage boys to use condoms every time. Get her on the shot or something along those lines to make sure there are absolutely no birth control f%$& ups.
Yes, he may be criminally responsible here, yes, 13 is too young and have sex and she shouldn't do it, and yes, birth control does not prevent STD's... BUT the one thing that your REALLY don't want is for her to get pregnant. Most other things can be dealt with more or less easily but a pregnancy at 13 really screws a kid up.
Unfortunately I know quite a few women who got pregnant as teenagers and 99% of them are screwed. Many had abortions or adoptions which screwed them up psychologically, they ended up depressed, taking drugs, promiscuous. Some got pregnant again on purpose during their teenage years to make up for the baby they aborted or had given up.
A few had their babies and will likely never make it out of poverty (they are in their early 30's now). So do NOT let her get pregnant.
Do not think you can "control" her. She will find a way. If you can get her some counseling, call the cops on her "boyfriend"... but do not think that that would put a stop to it or would keep her out of harms ways for good.
Get her on birth control. Don't wait.