What is the ruling on a Muslim saying: "O Helper of the infirm!"? And is the objection of one of the sheikhs to this phrase valid, on the grounds of restricting aid to the infirm?

There is no objection to saying: "O Helper of the helpless," or "O Answerer of the prayer of the distressed," or "O Rescuer of the afflicted," because these descriptions are mentioned as a way of informing about Allah Almighty regarding the vastness of His grace and the greatness of His generosity. They do not imply that He only helps the helpless, but rather that He is in greater need of help. And the scope of informing about Allah Almighty is wider than the scope of naming Him.