There's always the singular they. Works in every situation. Hell, even works if you don't remember whether someone told you on the internet if they were a guy or girl- just use they, and you're set!
Nope, not true. To wit:
You see a person that is new to your care, and you have not been introduced to. This person looks male, but is transgender. They clearly have dementia, because they are wandering abound lost, and are pulling off their pants and doing the pee dance. Unbeknownst to you, a family member is in earshot.
You ask the person, quite earnestly, "sir, do you need the bathroom?", because you are trying to avoid their public embarassement about incontinence.
Again, do no underestimate frayed nerves in others about touchy subjects, and where that can take things, quickly. Let's say the family member is the DPOA, and they have been trying hard to get this resident placed in a facility that will respect their gender preferences, and they hear this.
This is just toxic shit all around. You cannot just shout "hey you!" at the new resident, you are required by state regulations to address them with dignity and respect. You fucked up, but did so accidentally.
This DPOA has had negative experiences at other homes, and goes right for the nuclear option. (Going straight nuclear is very common.) we are talking full Facebook hate post, and all that.
"They" cannot be applied here. Unless you are told about the preference, (and communication is often very poor among staff), or you somehow develop telepathy to just KNOW, You have a 50% chance of getting it wrong. You also are doomed by trying to determine gender from body morphology, so your chances of getting it wrong are much higher.
It would get even worse, if there are more than two choices to pick from. Eg, if there were two additional gender pronouns, you have a 75% chance of getting it wrong at random, and with the above setup, that drops to even less.