If you look at last months thread I put some mods on for the ab moves if you have diastasis. I can also link you to another thread where I posted loads of exercises that you can do along side the shred to keep the separation closed.
Basically if you've ever had diastasis then you are at risk of it coming back as soon as you start putting pressure on the abdominus rectus which is the 6 pack muscles. Sit ups, crunches, planks all put pressure on those muscles. You can tell which exercises are the put pressure on ones as your abs will bulge outwards. The muscle which you need to keep strengthening and which reverses the separation is the transverse. It's like a corset all the way around your body. When you do exercised that engage the transverse muscle your tummy will go in rather than bulge out.
If you've got 1cm separation you're probably ok to do the shred but keep checking the separation (someone posted a link upthread) and if it starts to get bigger STOP. Ideally check it the morning after each workout. Try and do transverse exercises as much as you can, like pelvic floor exercises that you do whenever you remember, you can't over exercise the transverse. The one to do as much as pos is as follows:
Stand up, feet shoulder width apart, arms by side. Place one finger in your belly button. Now imagine there is a string from the inside if your belly button, to your spine and out a hole in your back. You're going to visualise someone pulling that string and pull your belly button in. Your spine shouldn't be bent or curved, you shouldn't be holding your breath and the top and bottom of your stomach shouldn't be moving a lot. You may feel it tighten in your sides and back - this is normal as it's where the muscle is.
Pull it as tight as you can and hold for as long as you can.
Once you've mastered that you can do a more complex version. Pull it in as before and now pulse it at the point of your belly button. Keep the muscle contracted the whole time and pull it as tight as you possibly can on each pulse. Contine for as long as you can.
The best way to prevent separation returning is to recondition that transverse muscle to engage automatically. ATM it won't be. So if you spend several days making a conscious effort to engage or contract that transverse muscle whenever you bend, lift anything, carry anything it will begin to automatically work with you other core muscles when you lift things, which will ultimately keep your core strong and prevent separation. If you think about it, what is the point of doing exercises to strengthen that muscle if the rest of the day you are pushing your abs outwards (which happens if transverse isn't engaged) whenever you bend down or lift anything or twist. That's all putting pressure on those abdominals and risking a separation returning. So if you can do exercises every day, retrain that muscle engage when it's needed you should have a really strong core and be able to shred without risking those abs bulging out.
Don't do what I did and think it's gone forever and go crazy with ab exercises as mine came a bulging right back out again. The 5 months pregnant look is not one I'm going for ATM 
I hope that helps.