I've tried cooking low FODMAP meals a few times (my sis has IBS), I am no expert, but I find it easier to use my regular recipes and find suitable substitutions for ingredients that she doesn't tolerate. I will often omit onion altogether or use the green part of a spring onion (if they are tolerated and you don't have any in the garden, you might like to start growing them!).
More and more people are following a low/no fodmap diet these days so there is a little bit more info around. Good luck, I hope the diet solves her issues