Agreed, context is everything. Putting someone on your Christmas card list is entirely a different context to declaring you have a crush on them.
To clarify:
”cake on a Friday”
”oh I’m going to love working here”
Absolutely fine
”cake on a Friday”
”oh, I think I love you! Ha ha ha”
OK, but a little bit awkward, unprofessional, over familiar
“cake on a Friday”
”Ooh, I have a crush on you”
Not ok, very awkward, very over familiar, very unprofessional.
On a personal level, as an employer, I’d be thinking twice about employing her. On a professional level I’d be concerned if someone who is going to be client / customer facing, blurts this kind of thing out in inappropriate situations. That kind of behaviour can also blur boundaries in employee relationships, what if she says something similar to a junior colleague.
The fact is, OP’s DH was flattered. Whether she was joking or not, it will have changed his personal view of her and the dynamics of their ongoing relationship. As PP mentioned, would they have laughed if Colin had said the same thing?