Try the following exercise program twice daily. It takes 3-4 weeks to make a significant difference.

Teach typical pelvic floor and TrA exercises in side lying, sitting, 4pt kneeling, standing... and encourage her to activate them when performing the routine.

Explain that it is advised that preganant women after 20 weeks gestation don't lay on their back for an extended period of time due to possibility of the weight of baby compressing the inferior vena cava, however, this routine doesn't require her to be on her back longer than 5 mins. It would take at least 20 mins to cause compression of the vein. If this were to occur, she would feel light headedness and the baby would become restless. Women are one their back for gyne exams and ultrasounds etc throughout their pregnancies.


In Crook lying:

TrA activation with knee rolling side to side, short of pain approx 1 minute.

hug knee to chest (around tummy) hold up to 30 secs, then straighten the leg for hams stretch. Repeat other knee.

lift up one bent knee and push against it with the opposite hand in an isometric fashion for 5 secs. Repeat 3 times on each leg.

bridging 5-10 reps with no hold (only if pain allows)

In 4 point kneeling:
Bottom waggle side to side for pain relief approx 20 secs or however long she likes

Superman core exercise with TrA activation (extend one arm and the CONTRALATERAL leg) hold up to 10 secs. Repeat 2-3 times on each side.

Legs wide apart, sink back so that her bottom rests on her heels.
Stretch arms out straight in front and place forehead on floor.

In sitting:
Piriformis stretch. Sit with right ankle resting on the left thigh. Reach hands around the lateral aspect of the right knee and pull diagonally towards trunk. Hold 30 secs. If she doesn't feel a stretch with that, try externally rotating the hip by pushing the right knee away from the trunk. (this is due to change of action of piriformis around 90degrees of hip flexion).

You should also trial passive stabilisation with a maternity/Wikipedia reference-linkSIJ belt. They cost between $20 and $45. To see if it works for her, find a way of reproducing the pain (single leg stance is often provocative), then put the belt on and reassess.

Piriformis massage in side lying is also a nice hands on technique.
Hope this helps