Diastasis Recti is the separation of the linea alba (connective tissue that connects the abdominal muscles).   Picture six pack abs—the cut line down the middle of the six pack is the linea alba.  Sometimes the linea alba can separate.  This most typically is caused from pregnancy with the expansion of the stomach and spreading of the abdominal muscles, but being extremely overweight and large around your midsection could create diastasis recti as well.   

The problem is, you need to be very careful about healing the separation of the ab muscles.  Big ab movements (even as big as a regular crunch or oblique twist with your feet on the floor) can be too much and potentially exacerbate the problem.  Remember you abdominal muscles are connected to this tissue, so when they move it moves.  You want to start moving them together so that connective tissue can reconnect instead of get pulled further apart.
Here are three great exercises to start with (so long as your doctor approves), to try rebuilding the connection of the linea alba:
1)      Abdominal Scoop (without moving the Pelvis).  Lie on your back, knees bent, feet on the floor.  Neutral (slight arch in the low back) pelvis. Place your hands on your lower abdominals.  Fingers pointing toward your pubic bone.  On every exhale, try and pull your abs gently away from your hands without rocking your pelvis back in space.   There are not bones moving on this exercise, only muscles.  To help, imagine putting on a pair of jeans that just came out of the dryer after you at Thanksgiving dinner.  Be gentle with yourself.  This is a subtle engagement.  You don’t want to pull down on your abs to the point where it prevents breathing.
2)      Pelvic Floor or Kegel exercises.   Same position.  On every exhale think of drawing your pelvic floor muscles up and in.  When you do this you should feel the same compression in your lower abs that you were just doing in the first exercise.  So this is like a double whammy.  You are feeling your Kegels and your deepest abdominals pulling toward the center of your body, but you’re making it happen from the base of your pelvis instead of in front of your pelvis.  This time the front is just coming along for the ride.
3)      Hand or Towel Supported Crunches: Same start position, but take a scarf or towel and wrap it around your midsection.  You are going to do what we can in Pilates Ab Prep.  It’s a crunch while holding a neutral pelvis.  So when you do this exercise the middle of your abs should not pouch up.  You should feel your abdominals flatten out without making your back cram into the floor.  Plus, you’re building.  As you curl up you are doing both the Abdominal Scoop from exercise one and the Pelvic Floor engagement from exercise two.  As you curl off the ground, you are tightening the scarf, drawing the abdominals together.  If you don’t have a scarf, use your hands—finger tips pointing toward your midsection and when you curl your head and shoulders off the ground, use your hands to guide your abs toward the center of your body.
Here it is:
Start: Neutral Pelvis, Knees Bent, Feet on Floor
In hale: Nod Head to Protect Neck
Exhale: Curl Head and Shoulders off Floor while pulling scarf or hands around waist, zipping jeans and engaging Kegels.
Inhale: Stay
Exhale: Release head and shoulders back down to the floor.
Do these exercises every day.
And if you can’t stop yourself from going to a class and doing abwork, at least bring a scarf or use your hands to hold in your abs when you are working.  And please, please, do everything in neutral—not imprint.  Which means you shouldn’t take your legs into the air because you need to imprint to do that.  Your abs will be better served keeping your feet on the floor when you exercise if you have a separated linea alba.
Check out this video to help you through the exercises.