My main point, however is that if you hire something you enter in to a contract with the owner who has a duty to supply what he states you are getting.
If he informs you that the sat nav is out of date, that is ok.
Erm no.
If you enter into a contract to hire something that is simply described as "an aircraft with a GPS" .... and you are duly supplied with "an aircraft" and "a GPS" then the owner has fulfilled their duty.
Much like if I sold you "an iPhone" on eBay and it turned up INOP. I never stated in the description whether it was functional or not, and you never asked... thus its your loss.
Do not read any more into a contract than is explicitly stated, because more than likely the only person that will end up with a red face is you.
Also, from an aviation point of view.... you as the PIC have the ultimate responsibility. So its up to you to RTFM and check for yourself the GPS is up to date..... much in the same way you check the fuel levels in your tanks.