Yep - my first thought was the same as Anns... it should be the ex telling him.
Probably, she won't - for whatever reason. I suppose you can let him have the papers served on him, and he can find out that way. It does seem to me that being in a rehab with counselors on staff - all day - might be the BEST way to receive such news.
You could warn the counselors it is coming, and then just have the wife send the paperwork along. No reason for you to visit until after he is served, that way, you aren't the bearer of the bad news, and he has time to absorb the information. Once he has absorbed the information, THEN he can take any next steps....
((hugs))