It depends - do you shoot in 2k, and you're just editing/outputting at HD? In that case, it's possible to export out a 2k file if they want one, and you're good to go.
If you're shooting in HD and selling it as 2k, I'd say that's pretty dishonest. While the actual pixel count isn't really that much higher for 2k 16x9, if you're shooting at a different aspect ratio, it can be substantial (an extreme: 4x3 footage, say a film scan, at HD resolution is 1440x1080. That's half the resolution of the same film scanned at 2048x1556. So there's a serious difference there.
If it's 2k 16x9 (2048x1150), the difference over full frame HD is about 13% smaller. I'd say that's substantial enough that as a client, I'd be upset, but on a technical level it probably doesn't make that much difference.
If you're shooting at 2k and just working at HD, there's really no problem with that, if you're giving them the 2k file. We do that all the time - our grading room is set up for HD monitoring, but we do 4k grades every day on that system. The 4k is downconverted to HD for monitoring. The color space is the same, and there's no real benefit to monitoring in 4k in a small room like ours. The final export is 4k, though, unless otherwise requested.