OP - I'm so glad that the police are taking this seriously.
Whether the 15 year old's Mum is doing the same thing, may be another matter. If I had a son who had done this I would be mortified beyond belief.
Someone touched on this earlier - the reasons behind why a 15 year old boy had posted such revolting words on FB. Well, in my opinion the answer is fairly simple: on-line pornography.
Accessed easily and mostly free, there are images and words out there which degrade and dehumanise women to a degree that I'm not sure we totally understand in terms of the short, medium and long-term effects on young people in this country.
Only recently, a 12 year old boy raped a 9 year old as he "wanted to do what adults do".
I'm 48 and there is simply no way that when I was 15, a boy my age would have made such vile, "porno" comments towards a lady in her mid seventies on a public forum, should such a thing have existed if you know what I mean. Porn was accessed by shifty-looking blokes; fifteen year olds would have needed a great deal of courage to ask for one of "those" magazines. Now - it's everywhere, not even managed and blocked by too many parents.
Boys are VERY sensitive to sexual images and the sooner we realise this, the better.
We are sitting on a time-bomb.
OP, I hope that your MIL is OK. This situation is beyond disgusting and I hope the Police scare the bejeesus out of this horrible little article. Yuck.