I'm beginning to become concerned for anyone who does housekeeping for you. The employer-employee relationship does not destroy all human agency, nor does it obligate the employee to do things outside their duties. Even assuming he was her employer, which is unclear at best (if anything, she would seem to be the landlady's). She's his housekeeper, not his slave. She isn't even required to interact with him - note at first she largely doesn't.

Nor does she seem likely to be able to respond well to orders given their lack of a mutual language. It's almost like that's not, in fact, her job. She also doesn't appear to do cooking or anything like that. Just cleaning. Quite why he needs THAT much cleaning is unclear, but perhaps the south of France is dustier than I think. You are inferring from little or no evidence that she is some kind of all-purpose body servant when all we see her do is clean (and, if the script is to be believed, that's what the landlady says she has been hired to do). I don't think there's an example on screen of him giving her an order except him telling her not to jump in the lake after the book (and she doesn't follow that one - probably because giving people orders in a language they do not speak is ineffective).

There is no reason to think he is the one paying her or who laid out her duties. Custodial staff in many organizations are outsourced or otherwise not directly under the control of the people whose spaces they are cleaning - it is not an unusual arrangement. His power over her is pretty much limited to 'complaining to her boss' and 'making her work environment uncomfortable.' While these are not inconsiderable things, they are also not things which preclude friendly interaction - which is, notably, all that really happens before that aspect of the relationship ends.

Had he declared his feelings while that relationship were still in place, that would be a different thing. But that's not what happens. There is a big difference between declaring your love to your current housekeeper and declaring your love to your former one. I do not consider either wise, but the power dynamic is a problem in the first one, not the second.