Why does she need to tell him at all? If she's permanently moved all the way to Las Vegas to cut ties to her piscine heritage, why doesn't she live as if she were human? If she's smart enough to realize her man might be upset by this news, why tell him? (Obviously she's made it work for 7 months)
And if they get married and have kids, and if the kids have tails? Should she just keep her mouth shut still and just Google the number for Charles Xavier's School for Gifted Young People? Or turn the kids over for medical research?
There are some things you don't tell people on the first date, however, they would seem to be a little past that point - probably to the point where even if he rejects her romantically, he probably won't sell her to the National Enquirer.
It would help if she chose a man who's a marine biologist or a fantasy lover, or someone who's already preinclined to be more okay with having a mermaid girlfriend than most. If he's just an average Joe who'd hate the idea of being with a mermaid, why did she choose him of all people and lead him on all this time?
But what if that wasn't what the mermaid was interested in, Fantasy lovers may be annoying, because of all of their preconceptions/misconceptions that she would constantly have to explain.
And a marine biologist? That's kind of like saying that humans should marry doctors because they understand our bodies. Besides, if he is a scientist, there is always the danger that the FOR SCIENCE mindset would force him to sell her out - possibly regretting it later, possibly still leaving them with a HEA, but the potential for conflict remains.
As is, she sounds like an unsympathetic character to be deceptive for this long, unless she's been probing him for clues that he might like this sort of thing, or hinting at her true nature. If I were a drag king and fell in love with a woman who thought I was a man, I'd cautiously probe her with questions to see how she feels about lesbianism or female cross-dressers. I certainly wouldn't lead her on for 7 months, telling her I was male, and then expect her to be okay with it when I reveal, "Tah dah! Been a woman all along!" That would be a real asshole thing to do. I'd imagine her trust would be shattered and she'd dump me, and rightly so.
I present to you an alternate scenario. Bob is a young transguy who "passes", whose HRT and naturally high testosterone levels never let anyone see that he was in fact, raised as a girl. However, because of his family's reaction to his transition, and the fact that he lost most of his friends as well, he has moved clear across the country to start a new life.
He meets a nice young lady at...I dunno, let's say the library, they hit it off, they start dating, and genuinely come to like each other, they do in fact, fall in love. Now, Bob hasn't told her that he's a transman, because of the fear of rejection (due to being burned by his friends, his family) and is terrified that the one good thing he's found in his new life will also leave him.
That, and most importantly, he wanted her to get to know him, to know the real him, before disclosing what is really an unimportant physiological fact.
The mermaid really is no different, she wants her boyfriend to love her for who she is, rather than wearing a sandwich board to club, saying "I AM A MERMAID" and letting that be the controlling factor of the relationship. I mean, she presents this fact straight-up and she may get someone who has a mermaid fetish, and lets that be the dominant factor in their staying together, rather than say, caring about what kind of music she likes.
Make sure you don't write your character as a selfish merjerk who's unconcerned about the feelings of her partner!
If she didn't care about her parnter, wouldn't she just drop the bomb and let the pieces of their relationship fall where they may?
