Unfortunately, our son did the same thing, changed his name to our DIL's last name. Really no reason was given to us, but that conversation was one of the last we had with him some 3 years ago.
As with so many others in this situation, we were a close family, or at the very least, a normal family, not perfect, but not crazy dysfunctional either. That is, until our oldest met our now DIL. They met in college, his first GF, and in some ways she was good for him - he became much more focused on his studies and his future. In so many other ways, there were red flags in all directions. Her father walked out on her family when she was 5 or 6, and from what I could gather, her mother and sister are very bitter toward him, but she's embraced his presence in her life. She's close to her mother and sister but her continued relationship with him has caused some contention in the family.
He brought her home for a school holiday, only 4 months into the relationship. At the end of their visit, we held a family meeting to discuss future holiday plans, and as she had only dated him for a few months, we didn't include her. Honestly, as our oldest, she was our first experience with one of our kids dating, and we just went with our gut. Apparently, that offended her horribly and things went south from there. Some family members have since told us that we should've included her, but I'm not sure I would've done anything differently.
A year later, they decided to get married - a courtroom wedding they said - to which we weren't invited. Supposedly her family wasn't invited either, but we later found out it was a real wedding ceremony and her family attended. A few months later, we found out after the fact that he'd changed his name to hers. With the history of her father leaving, my husband and I can't figure out why his name is so important to her, or our son.
Phone calls became fewer and fewer, and visits ceased. Except for that family meeting, we tried to be as inclusive as possible, and made every effort to befriend and love her, but by the time we realized what was happening, it was too late.
I don't want to discourage you, but your son has chosen her, for some reason, and from my 3 years apart viewpoint, your best bet is to keep your mouth as closed as possible. Don't give them more ammunition to find fault with you. This doesn't mean you can't continue to love him and hope for a relationship, but you really are in a da*ned if you do... if you don't dilemma. There's little you can do about the name change, his choice of wife, how he lives, etc... so my advice would be to make the best life you can with the friends and family around you. He needs to figure out what's an acceptable life, and he may have some hard lessons to learn. And they may take years for him to learn them.
In retrospect, while it would've been painful to attend our son's wedding, I nevertheless would have done it, if for no other reason than to show our unwavering support and love for him. Period. (But smile and keep your thoughts about things to yourself!)
Sending you hugs and prayers for wisdom...