ANDY'S ANSWER: I cannot imagine what kind of parent would be promoting their 14 year old daughter's sexual activity. There is something seriously wrong with that scenario. Have you spoken to this woman? Are you sure she is encouraging / enabling the relationship? If so, I think I might be tempted to report her to Social Services. I'm fairly certain that given their ages, she could be criminally charged.
In the meantime, have a good, long talk with your son about safer sex. He needs to understand that there could be consequences that will affect him the rest of his life. Just telling him not to do it isn't going to cut it. Make sure he's educated so you aren't a grandparent by this time next year.