Are you asking about general exercises that are safe after delivery, or are you talking about fixing diastasis (where the connective tissue that holds the two sides of the rectus abdominis together rips)?
If the fascia is split, it won’t grow back together, but there are some exercises you can do to help train them to work more as a team. I just watched a video from IDEA 2014 about pelvic floor and core training, including a discussion of diastasis. I’ll see if I can find it and link it for you.
I usually work on awareness first, helping them to connect with those muscles that were overly stretched or potentially damaged. Small, gentle movements to re-connect can be very healing. Work on unloaded activation of the TVA (transversus abdominus), pelvic tilts and contract-relax in the area. Progress as they feel comfortable. Some people think that over-contracting the abs can cause them to protrude more. The jury is out and I always go for awareness and communication with the client about what they are feeling.
Here is an anatomy blog about the TVA I wrote and a video about the pelvic floor muscles. I have a TVA video coming out soon, keep a watch or join my newsletter to receive it in 2015!