Tell her what kind of man she should look for when she's older and tell her love is not like in the movies.Also, tell her how beautiful and smart she is everyday because girls need to hear that, especially from their dad or some sort of male figure. Whatever you do, support her decisions and don't criticize. Only support, this doesn't mean just for boys, try to support her in other interests that you think are appropriate in hopes that she will get more focused on music lessons or sports than on the guy.If she were older, a 3 yr difference wouldn't matter much but a 16 yr old is too old for a 13 yr old! let her know if this boy doesn't want to go 'anywhere' just to be with her, he really isn't interested in 'her', just her body. You get to know him, you can give your daughter a chance with him and if he doesn't like it he wasn't worth it, and they learn moral lifestyles, lets your daughter know she is worth a little trouble, if this boy doesn't think so another one will.Also, I'm not sure how she has been actually "dating" him and you didn't know till yesterday? After you get to know him tell her YOU will decide if he is good for her or not.
I know things have changed a lot but I think that's too young. (Check with leader to make sure they go to the classroom) Then on Sunday have them go to Sunday school together and church service.
I am not trying to scare you needlessly but you need know this a very serious problem.