What is the best exercise to target the multifdi with someone with a herniated disc that is without symptoms?

Pretty much everything that engages the transverse abdominis as the multifidi always co-contract with the TA. That means an exercise in a neutral pelvis to avoid getting the rectus and the obliques involved. To teach that I have people hook-lying, help them find a neutral spine and may instruct them to imagine lifting a leg while maintaining neutral. This would be an entry level. I may eventually do this on a roller and increase complexity.