Needpeace.......if I were in your same shoes, I would be inclined to give him his space. If it is the cae that there is a romantic relationship between them, it will reveal itself if you give it some more time and space. Lol....he isn't incredibly clever at it, if he actually used Her birthday. This is under the theory that, given enough rope, they will hang themselves.
The more you act suspicious and play detective, the more he will try to "hide" it.
If it is only mutual support.....then, nothing is lost. If it is some sort of rehab romance---those relationships almost always self-implode...and, tend to be accompanied by relapse when they do self implode.
Does he devote any time to a program...like AA? Does he have any time with a sponsor?