Pilates generally makes the diastasis recti worse - time and time again. There are way to many twisting motions which SHEAR the diastasis making it larger, and jacknifing up and jacknifing down motions (rolling like a ball, pilates 100, corkscrew, teaser) not to mentions many more. Also crunches are a BIG no no.
The Tupler technique is what I use with my clients, and splinting - sounds like M. REALLY REALLY needs to splint - and that will prevent new hernias from appearing - you really have to take precaution with that connective tissue in between the recti at 10 fingerwidths and splint RIGHT AWAY.
Time wise REALLY depends on several things: her technique, how often she does the technique (she would need to work up to 10 sets a day of 100 (about 30 min a day) of the seated tuplers at the advanced level), how often she splints, if she puts the splints on, if she avoids the compromising movements - like the ones mentioned in pilates - also getting in and out of bed correctly, being careful to bring the transverse to the spine when she holds the kids, her mind/body connection with the muscles, and her consistency.
At 10 fingerwidths (13 cm) the mind/body connection of the muscle might be very poor - but I would splint right away, get the book "lose your mummy tummy" and make sure the technique is 100% accurate, and she could probably make progress where surgery wouldn't be needed.
Julie usually tells people to expect 9 months - I have never had anyone take that long - 2-3 months usually - but 10s can be a little trickier - I would give the full 9 month allowance and if it happens quicker - a pleasant surprise.