The Tupler Rehab splint is super low-tech. You can use a scarf or piece of fabric that is approximately 5 feet long and 6 inches wide. Hers is made of terry cloth I think. She has you wrap your belly tightly with that to pull the halves of the recti together. Then, her rehab protocol basically involves breathing exercises that have you pull your belly button in toward your backbone, planks and plank-type maneuvers. NO pilates 100 type moves.
She has you roll to your side to get out of bed. Anything that looks like a crunch is avoided while you are trying to encourage the muscles to come back together. She'd probably want to splint her abs for a couple of months right after the baby. Tupler claims it can heal mild cases of diastasis, and minimize more moderate ones. I'd say it's worth a try. Even if it brings her split abs closer together, maybe her tummy will heal flatter. The compression garmet may help, but I wouldn't think it would pull the muscle together as well as splinting.
My experience. After baby number three I had an outie belly button. I had diastasis but I don't think it was severe. I could feel a distinct split down the mid-line when I did a crunch. My waist was about 24 inches. After lifting and doing LOTS of ab work my waist spread to 25 inches. At first I thought I'd just thickened up the muscle on my back and abs. NOPE. I had pain at the belly button when I laughed or coughed, and when I did leg raises.
The outie thing was much more pronounced. The diastasis leaves NO support for your belly button. It's the biggest cause of umbilical hernia in women. So, I had an awesome surgeon and when he fixed my hernia, he also sutured up my abdominals from above my waist, all the way down to the pubic bone. My waist went back to 23.5 inches, which is about where it was before kids. They will tell you the diastasis is cosmetic, but it is functional. You have nothing but a thin layer of facia holding your guts in. My surgeon was very cool to fix mine and call the whole thing a hernia repair.
I bought the Lose Your Mummy Tummy book after my surgery, because a PT recommended that I be careful not to do stuff that would tear up the sutures. MOST of her book is just really basic stretching and BW exercise like learning to squat your BW properly. I was sort of disappointed in that, but it did have some helpful tips.
Note: Her book makes only a passing mention to women who have problems getting their pelvis to heal back together properly. For pubic bone separation, they apparently also sometimes try splinting the hip area, and advises walking sort of like a mermaid. Also nothing that spreads your legs wide for a couple of months, use your imagination there.
You don't want to stand up while lifting something heavy, make love with legs wide, sit cross-legged, step over a child-gate, or even take large strides. In other words, nothing that would encourage the pelvis to separate while it's healing. I don't know if that's possible. That problem sounds like it can be especially painful. Best of luck.