Need more details.
Crucially, yes, was this person pre- or post-op? While I’m supportive of TG individuals, I’m also supportive of women who have survived sexual assault. I can see where being naked in a space you believed to be “safe”, and then unexpectedly and unwillingly confronted with strange penis (whether it ensues or not) could be extremely triggering for someone there. I personally am naturally comfortable with being naked, BUT this natural inclination is greatly hindered by an emotional reaction that clothes are protection against assault. If there’s no reasonable chance of assault (at home with doors locked, in a well-populated women’s locker room or wet spa), I’m fine, I don’t much care. If there’s some chance – no matter how irrational – I panic. This is not dependent on the penis-owner’s behavior, intent, or how realistic the “threat” actually is, I will feel threatened anyway. It’s not something I have control over, and I would actually have to leave the spa ASAP. Then they’d be faced with giving me my money back. Of course, if post-op, I doubt I would notice, and if the place had made it known that naked facilities were co-ed, I wouldn’t go in the first place (and no harm, no foul on that). (aka, what doreen and Septima said.)
This is a real problem, given how common sexual assault is. I don’t have a good solution for a pre-op customer, since they’d probably face the same sort of fear-of-assault going into the biologically-same gender-opposite locker room. But a business can only hope to please most of the people most of the time, so it might be a little unfair to tell them that the majority of their customers should suck up their discomfort to accommodate one, relatively rare, customer.
Now if it’s true that the staff were unjustifiably rude (as in, the TG customer was not trying to make a scene) and/or they didn’t offer a full refund of the entry fee plus any services purchased but not received, that’s a problem.
Diosa-- I’m pretty sure I’ve been to the spa WhyNot is talking about, and IIRC bathing suits are prohibited in the wet spa area. They probably don’t want threads and fabric shreds clogging up their filters.
You could change in a bathroom, put on the provided cotton shorts and tee, and skip the wet spa in favor of going straight to the (clothing-required, co-ed) dry spa area. Of course you’d have to be okay with skipping that entire part of the experience.