I am so sorry that she is trying to hurt her child and your son this way. She sounds like my exsil in many ways.
You have gotten excellent advice here. The MOST important things are to document, document, document, to have a top atty even if the entire family must pitch in to pay for it, and independent witnesses who SAW and HEARD the things she does. He should check the bank acct info to see where she is spending. I would personally hve someone who is not a friend she knows of become her fb friend (a guy fb friend if possible - even if the person is a woman) and they should print pics of her doing things and of her posts. She problem won't show it to your son or you, so someone she doesn't know who sounds approvin g is the person to do this.
Please don't be doom and gloom. Keep as positive as you can, and let grandson stay as far out of things as you can. The biggest reason to never criticize or talk badly about your now-daughter in law is because he knows or will eventually know he is half mommy and half daddy. This means if mommy is bad then he is half bad at least. Kids think this way. It isn't easy, but keep talks about this out of his earshot and urge son to do the same.
I would be tempted to get one of those spy cameras that can be put on his clothes or a toy or that looks like a toy and send it with him when he is with mommy. But a private investigator would be a better thing, esp as far as the courts are concerned. the atty should have advice on this.
It is NOT automatically the mother any more. The courts in our state give shared parenting. Period. You must prove extreme abuse to get anything else. This is apparently the natiowide trend as the middle of OK is NOT the most progressive part of the country.
Above all else, get documentation. If now-daughter in law's car has gps, or if she and son have a joint cell phone account, son should try to check these. He can likely add gps tracking to her phone and check it online or via his phone to see where she is partying.
I hope and pray that the best thing for the child happens. I do think that not being able to contact his child is osmething the courts will frown on badly. He needs to make sure that now-daughter in law talks to their child whenever she calls, and maybe even start having their son call every night that he is with Daddy so that he can say goodnight to mommy. The court will like this, and documenting the times when she doesn't answer or the noise in the background is of a party or whatever, could be invaluable.
Son needs to figure out how to record phone calls, and to save all emails, etc... and keep super detailed details of what is going on.