Definitely say something. I monitor both of my kids (13 & 16 yr olds) FB pages and I kinda monitor their close friends too. One of my DD's friends added some creepazoid from Indonesia who is 60! I sent the message through my DD to get her friend to delete the guy and watch out for who she adds from now on. I can not believe some parents don't pay that much attention to what their kids are doing online.
I hope your inlaws don't take it out on their DD but have a good talk to her about internet safety.