I feel so sorry for my nephew?

My nephew is 17yrs old and he has a great girlfriend that he adores completely, he is a very good looking young kid and a few girls around the town always try and cause trouble between the two of them. So tonight he was at basketball with his girlfriend and I and one of the girls in town that is jealous of them both had told stories to her mum about him trying to flirt with her, and guys she is 13 by the way, I have heard him say a million times that she wouldn't leave him alone. So tonight her mother walked up to him and in front of about 20 people started abusing him for trying to crack onto her daughter and called him scum over some thing that he was ment to be said to her on facebook. So when we got home tonight I made him show me all the stuff that she had written to him and he was not at all in the wrong, he showed me all his text messages and not one thing he said was wrong at all, the wwhole time he kept saying leave us alone, your 13 go away little girl and so on. Now her mother is causing trouble for him and he has sone nothing wrong, I personally think I will print all the things out from facebook and mail them too her mum so she can read what her daughter has been saying, I'm just worried that this will cause more trouble for him. What would everyone else do??? Oh and his parents are seperated and he lives with his nan and pop and we treat him like a son if anyone was going to say let his parents deal with it!!!