First, the girl definitely needs to tell her boyfriend about her problem with her mom. If he has an issue with helping her in her situation, then she needs to know that because he will not be the right guy for her in the long run. Couples need to work together to solve problems and that is a life skill that will serve them both long term.
Second, the mom has to be clear about her expectations regarding her daughter dating. Is there an age limit for her? If so, and the girls is below that limit, the girl really needs to follow her mom's rule. However, that does not mean that mom's expectations cannot be changed, with proper, honest communication.
It sounds like lack of communication is a recurring theme in this situation. Blatantly stating that "all guys just want sex" is too generalizing on the part of the mom, more needs to be said to the daughter here. And hiding her problem from her boyfriend shows a lack of communication in the relationship on the part of the girl.
The best thing families can do is have open,honest conversation---about everything.