I got out of rehab a month ago. The work and meetings are very serious. When you're cooped up for a month learning how to be sober, it is only natural to make friends and enjoy some levity. There are some people who are in rehabilitation by force and don't take advantage of the opportunity but plenty are serious. I have 54 days today and a big reason for that are the friends I made in rehab. We talk on the phone or text and encourage each other.
I can understand your fears but don't jump to conclusions. Hopefully these friends will be part of a network of support. You will remain his key supporter. Good luck