1. All housemaids going to Saudi Arabia are recruited through agencies based in the country of the housemaid, with a branch in either Riyadh or Jeddah or sometimes Dammam. Find out which agency she was recruited through (this is easy - her family back home will have the contacts of the agency in Manila. If they do not, you will usually find a copy of her contract with the passport department).
2. Contact the embassy of the Philippines in Riyadh, or go there personally. It is within the DQ in Riyadh, real easy to find (get into the DQ using the main gate, drive straight until the third roundabout, take the third exit on that roundabout, drive for 50m, you'll see the embassy on Umayah Abu As salat street). Pass on the details of the recruiting agency to the embassy, they have a liaison officer dealing with domestic worker issues. The embassy will contact the agency and get hold of the contact details of the employer. They will usually demand to visit the girl, and a threat to involve the police will typically result in the employer cooperating.
3. Once the embassy gets hold of the case, they will solve it in a couple weeks mostly. Stuff involving government intervention doesnt move very fast in Saudi, and the fact that the housekeeper is Filipino means there will be no rush (tough thing to say, but in Saudi your nationality determines how quickly the government responds to your issues. Filipinos are right at the bottom of that response pyramid).
4. The alternative, which is more risky, is to steal the girl and take her to the embassy yourself. This is, needless to say, very dangerous. But, if she has a phone and has Whatsapp or Facebook, she can send you her location using either chat program. You can then agree with her on a time and date when her employer is out, drive there, she walks out, you pick her up and drive her to the embassy. Very risky, but I've seen it done a few times.