I guess you can say she's not really used to the capital so Ian decided to take her in... like how ppls stay at a friends house while traveling somewhere.
If she worked somewhere else she'd have no reason to bump into Ian, and she doesn't have the qualifications to work in a restaurant.
Rather than Ian taking her in to care for her or anything it’s more to do with the fact she
• was previously employed as a maid (they can pull strings but not THAT many, at least Ann worked in the kitchen, that’s why she’s in a restaurant/cafe now)
• they are helping her find her family so sticking close for the information makes sense (or at least enough that she didn’t push for something different)
• the detective mentioned that Ian wanted her working at his (he likes her) so since she isn’t familiar with what or how many string they can pull she might think, we give you this job or nothing
i do like that there's a plot, but i think it would've been better if kate had gone to work somewhere else, with anne or something