Hi B
Well done hb. You could be right that his addiction has been the most divisive part of your relationship with his parents – but as parents in denial of their part of enablement I think you need to keep your barriers up.  
I know from friends that they hope that by ignoring their daughter’s addiction is will go away – in fact it doesn’t exist does it!    They appear (on the surface) to be happily standing by, watching her with her new man because you never know perhaps it will be different this time!    Unfortunately when she does go home there is always a bottle or two cracked open to celebrate and their daughter is an *********.   Naïve to the nth degree but sadly, I think, not uncommon – they couldn’t see that they had to change. When I talk to the mother she tells me all the wonderful things her daughter is achieving – all told to her by her daughter, I just want to get away because I am sadly aware the daughter’s alcoholism is active and they are hurting her by not offering a safe haven that she can trust – she cannot talk to ‘them’.   I was the one they turned to when they first found out their daughter had the problem, although they didn’t know that I had any idea about addiction. When I told them I had knowledge, they cried with relief and we spent hours together talking about how and why.   Since that night it has never been mentioned again – in fact the conversation is strictly taboo and she never asks about my CG.
I am looking forward to hearing how your conversation went yesterday.   I am hoping that it was calm and that your husband and you got the support you both need.  
I suspect that whatever happens you will still need to keep a close eye on his mother, her controlling ways will be hard for her to change and she may think – why should she change when she has done nothing wrong, which is the hardest thing of all to accept.  
I know you must be very proud of him but it is important not to over-praise. I learned this from my CG.   He said that ‘praising him for doing the right and decent thing was not right – other people didn’t get praise for being ‘normal’. Praise comes in a different form I think with a CG – it comes in them realising that they can trust us and I know it hard for them to do so. 
I like the fact his mother showed concern for you because you deserve it.   It will be good to hear that in the cold light of another day they have accepted what they have been told and are acting appropriately.
Your FIL is one of a string of people I have heard say ‘I’ve always gambled but never had a problem’ and I always want to say ‘how jolly good you – unfortunately it is not so easy for others’. The friends I mentioned above like a drink, in fact quite a lot, so why can’t their daughter stop like they do?   It is a long haul I think for those who love a CG to realise that unwittingly they could have enabled the problem and not been responsible.   With knowledge, in my opinion, we owe it to a loved one to look at our own behaviour without thinking it is just them that has to change.  
I hope all is well