If your family doing the guilt thing, then you need to address them, not the ex-in law.
Let them know that the book is placed with a scam outfit that rips off inexperienced writers, and while you want to support his writing, you don't want to pay money to pack of con artists.
It costs PA 3.00 to print a book and most books ship at media rate for about 2.50.
Ask them how much they paid, then let them know anything over 5-6 bucks went straight to the publisher, not the writer.
If you think it will create conflict, then do NOT invite them to Google Publishamerica + "complaints."
Right now, he's proud of being "published" and not likely in the right frame of mind for the truth.
That will come when he gets his first royalty check --probably this August-- and smells the coffee. He will need support and encouragement then.
Be prepared to accept that he may not smell the coffee.
But be firm and diplomatic with the family. Emotional guilt is a nasty weapon, but only if you allow yourself to feel it. You're doing nothing wrong by saying "no thank you."
Years ago I had the opposite happen to me - the opposite to what I see in almost all PA threads. I had my family try to pressure me into sending my stories to PA. I received written material from them. (My older sister would print out stuff she 'found' on the internet and give it to me.)
My family also sent me information from every vanity publisher they could find. (Many of them advertised in the back pages of various magazines.) I was constantly getting stuff in the mail and throwing it in the garbage. This began pre-internet, so it's a wonder I didn't fall into the pit and get buried by the well-meant intentions of cousins, aunts, uncles, sisters, distant relatives, etc. I so remember getting this stuff and chucking it and my mother saying, 'shouldn't you at least read it over?'
When I finally said, 'You got $500 for me to publish something?' she let up and the material stopped coming.
But I distinctly remember PA sending me things, too, and one of my sisters saying, 'Well they sound okay. Why not send something in and see what they say? If they reject you, you can at least see what they say.'
I credit the fact that I am and always have been a bit of a contrarian (more than a bit, I think) and I stood my guns and threw all this junk away.
I hope I didn't go too off-topic for this thread, but I have been avidly reading all the PA threads (and similar) on AW for a few years now.
But to the fellow who's feeling pressure from the family: Family pressure can be brutal. If you don't succumb they can pile on the guilt. Stand your ground. Be courteous about it, if you can. But if you can't, so what? Speak your mind. And the advice about getting it at a book store - brilliant. (Just make sure he doesn't run to the closest book store and order it through them for you.)