Hi.
Yeah, I had that. (ten pound baby and c-sec). First, it's really important to know that a lot of normal exercise which is usually great for you, is the worst thing if you have separated muscles. Until it's fixed, avoid any usual ab exercise like crunches, planks, pushups, any twisty moves. Even high impact cardio can worsen it. If you look it up, you'll see articles that list the slew of exercises to avoid.
Okay, so to be on the safe side, once I realized I had it, I simply walked 30- 60 minutes daily to help with losing the pregnancy weight gain, and also did specific exercises for the diagnis recti every day. You can find vids on YouTube.
After about 3 months of doing the diagnis recti exercises, the gap closed. Then I started up my normal exercise again. (once the gap is healed, THEN things like planks are fantastic for toning the abs)
Good luck!