It sounds like she wasn’t going to have sex with him anyway, due to being swamped at work, so I don’t feel it was a lie. If they are newly dating and she doesn’t yet feel comfortable discussing a potentially embarrassing, private health matter, that’s perfectly understandable and she found a tactful way to avoid the subject.
However, I feel like there should be a point in a romantic relationship where the two people feel comfortable enough with each other to discuss these kinds of things. If they aren’t newly dating, say they’ve been together for enough time where she should feel comfortable divulging a yeast infection, then that’s a bit weird and more along the ‘lie of omission’ line. I personally would feel a teeny bit hurt that she didn’t tell me the full truth. Still, I feel like the work would have kept her from sex anyway, so it’s not like she lied to get out of it or anything.
I know I’m not a guy, but when I’m in a relationship with a woman, I care about her and her health is my business, you know? If there’s something going on, I would like to know, so I can support her, help her deal with it, or even just be understanding. It’s admittedly a bit different when you have two girls in a relationship and I like that. The ones I’ve dated have been easy to talk to about various health issues and it’s really, really nice to have someone who understands the various female conditions that get in the way of sex and other things.