I think you should have to approach him directly on the matter as indirect way might not be helpful. However, do not tell him that he has flirtatious nature but tell him that he needs to talk less to the maid as his talking to maid sends wrong signals to her and let him know this was the reason why previous maid left the job.
If he does not get the hint then talk to him directly even if it hurts him as it is important to let him know that his behavior is not acceptable and he should not behave in such a manner. He needs to understand that maid is also woman and she has self-respect too. It is her job and allow her to do her job with utmost respect. It is time for you to take tough stand since your father is in wrong business.