There's no such thing as safe when it comes to sex. There are many members on this site that got pregnant even with 3 different forms of birth control. To say that using protection will prevent pregnancy is not accurate advice. Pregnancy can and does happen even with the best contraception.
The bottom line is this, she's 15, and in most states it's illegal for a 15 year old to have sex. Her boyfriend is 17, he could be charged with statutory rape, spend the rest of his life as a sex offender.
I guess my question is this. How much does she love her boyfriend? Does she love him enough not to put his life at risk? If she does, she'll wait. If she goes ahead with this anyway, not caring about the consequences to her and to her boyfriend, than she's definitely not mature enough to be a mother which means she's not mature enough to have sex.