Usually clinics that have inhouse translators have a different price list for procedures. Since they have to cover the cost of the inhouse translators and more. Also the inhouse translators are most likely to persuade you or push you to get more done than you may have originally planned, even if the prices are reasonable. As far as I know they work off commissions so you can see where this comes from.
Regarding the many "friends" out there I see on this forum. You need to understand that nobody will work for you for free. If they are not charging you, they have a back deal with the clinic. The price point will be about 2~3 times higher if you bring that "friend".
Best thing to do would be to go to the clinic that has a inhouse interpreter and haggle best you can, or hire a local freelance interpreter or a interpretation agency that charges you up front.
If you are afraid of negotiating on your own, or if you want help with airport pickup, hotel bookings, post-op care , etc. You can go with the "Friend" which is most likely a unlicensed agency, or a interpretation agency that charges you up front and make them sign a waiver that they don't have a deal with the clinic that you are going to for a rebate. (If the company is a properly operating place, they should already have this and offer this too you even before you ask for it.)