It depends on how you define "stress". Any kind of exercise puts stress on the muscle when it comes to physical training. I'm thinking you mean pain. There are many bodyweight exercises that you can do that engages the abdominal wall without adding discomfort. Using an exercise ball and doing crunches is a great exercise but make sure the individual is crunching upward and not outward or forward because many people cheat and use their neck, which will lead to that "stress" you're talking about.
The plank exercise is also a great exercise as long as they aren't using their lower back but tucking their stomach in and squeezing the glutes, which will force the individual to use more of his/her abs and not the stabilizers like the shoulder joints and the lower back.