If anything, the paranoia from both of them killed the deal.
We all know that if he'd blindly sent the money just because she asked and then she ghosted on him, a number of people on this board would be telling him what a fool he is, he should've done better research, etc. Well, he did his research and could not find any indication that this provider ever asked for a deposit before. That gave him reason to pause. I don't blame him.
Let's not forget that according to the OP the date was booked and confirmed days before she brought up the deposit. She's trying to change the deal. Whenever a situation comes up around here where a monger tries to change the deal after confirming, everybody jumps up his ass, and for good reason. Whoever she is, I don't blame her for being nervous. Apparently something about this guy or this date made ears perk up for a NCNS. That's the bastards who pull NCNS ruining it for everyone.
I think she should take whatever it was in this case that made her nervous and add it to her site; "in this situation I need a deposit of X amount." She also needs to figure out a better way of collecting the deposit.
Again, I see her side of it, she's worried about lost opportunity and money, and she doesn't know this guy. Maybe someone she knows is reliable asked for the same time slot and she didn't want to tell him no just to end up sitting around with her available now flag up, waiting for the phone to ring. I get it. It was not at all unreasonable for her to want a deposit. Asking for it five days after she confirmed is out of line, but not unreasonable. It's also NOT unreasonable for him to ask for some assurance so he can feel comfortable about it, especially considering she doesn't normally do deposits. Then the fact she was prompt at first, but changed to taking days to respond to him; anyone would've been at least a little nervous. If a five minute chat on the phone would help him to not be a nervous wreck from now until the moment she walks through the door, then so be it. Considering the coin a Vegas gal is going to make on a three hour date, refusing to give him that is unreasonable, IMO. Ghosting him instead of simply saying something like "hey it looks like we don't quite have a meeting of the minds here, I'm not able to speak with you on the phone, perhaps you should find another provider" is just rude.
Most likely if he'd sent the money everything would have been fine, if she's well reviewed and established as the OP said. But that's not a guarantee. We all know plenty of providers with huge, solid reputations have pulled shit and screwed guys over for some reason or another. It's rare, but it happens. They know their popularity will win out over one sniveling monger reporting shenanigans. For pete's sake, there are providers with a history of outing guys that still have them lined up to see her, real full names and all.
I haven't ever paid a deposit, but I recognize they have their time and place. I am not dead set against them, I'm not one of these guys running around saying never ever pay any deposit. I'm sure I'll pay one eventually. In THIS situation, I think I would have done pretty much what the OP did, and I'd feel just fine about it. I don't think he did anything wrong. She's told him more than once she'll call him but didn't, and then went radio silent. She's demonstrated that she has at least some level of flakiness. It's very unlikely she's pulling a premeditated theft of a deposit, but if she's flaky then there's another layer of risk right there.