How to deal with new girlfriend? You don't. Unless of course she's abusing your son. I know it sucks but the only thing you can do is NOT let it effect your relationship with your son. He's 14. Give him some credit for seeing things as they are all on his own. The less you show it impacts you the more likely he is to be honest with you in the future. He's obviously already on the defensive by lying about her car so you need to make it a non-issue right away. Just ignore her because that's best for you AND your son.