Just about every exercise is contraindicated for someone and perfect for someone else. So, while I don’t personally use or recommend them, I wouldn’t automatically say that no one should use it, and I wouldn’t automatically refuse to buy one for my club.
What I would do, whether we had a roller at our gym or not (we do, but I don’t use it with my clients), is develop a culture within my clients to ask a trainer how to use a piece of equipment if they’re not sure what to do or how to do it properly. i.e. limiting range of motion by rolling towards a wall that is within their shoulder, back, and core tolerance.
If you choose not to buy an ab roller, that could be a good lead-in for discussion, education and rapport. “We don’t have one right now. Can I take a few moments to show you a few other exercises that would target your core?”
PS, if you don’t have an ab roller and you want to make one, an Olympic bar with round rubber plates on each side makes a nice ab roller and has more stability since there are two points of contact with the floor.