My mother would like to know how she can treat her bloated stomach, she is 55, and gave birth to triplets 18 years ago. And used to be very slim.?

Congratulations. If you are one of the triplets asking for her, your mom did well not only in the pregnancy but also in raising you! now it is time for her to take care of herself :) the first step is to have gyn exam to find possible cause for bloating and get advice re exercise program and healthy diet. Next she could lay out a plan for how, where, when to work out. Then you all can help free her up to do it!
Core strengthening. Core strengthening exercises using a pilates and/or yoga regimen are fantastic for proper posture, body mechanics, balance, and flexibility. Of course this is the recommended approach. However an abdominoplasty (tummy tuck) may provide another option for correcting separated muscles contributing to a bloated feeling as well as redundant skin.