What do I do about my 14 y.o. daughters relationship with a boy that is going way too fast?

Daughter head over heals about a kid whom she has put high up on a pedistal as he has given her a friend network and companionship which she lacked. She has told me that the two of them had a brief encounter which she regrets but all the same.... She is a tender hearted girl that seeks approval and this kid makes her feel special. I am sick and do not know what to do.... Do I lay down the law and say.. you can't see each other and possibly fuel the relationship or ??? My daughter appears to me to have a low self esteem and doesn't see how dangerous this can be... She gets a's and b's and he is poorly motivated with d's. There is no drugs or alcohol involved... She is petite and beautiful but doesn't see it as she had been heavy in younger years. He is a heavy goofy kid that thinks she is the best thing ever... Do I go to the other parents and my ex or ??? I truly am scared and don't know which way to turn....