A friend's domestic helper has taken to bathing in their bath and using the lady's toiletries and comb etc.

What is the kind of thing one should say to indicate this is not desirable, or do people feel it is fine and she should be left to continue to do so?

  , Well, maybe she was used to sharing such stuffs with her folks back home. Simply tell her to use the common bathroom and the toiletries therein and that Oga & Madam's own is restricted to them. However, if the common bathroom is not in good enough order you may need to make it 'usable' and make sure the necessary toiletries are actually contained therein. If she now repeats it then you can discipline her your own way