OP: you did the right thing. Always assume every gun is loaded. Always verify for yourself that it is unloaded and the chamber is clear REGARDLESS of what anyone else says. Don't be so surprised though; some gun owners - just like car owners, lawn mower owners, home owners, business owners, etc. - are idiots. Use your own brain power instead of relying on theirs and you'll do alright.
In my experiences, having someone say "it's empty" is just as dangerous as someone handing a person a loaded gun. Yes, a loaded gun is ready to go and inherently more dangerous in that respect. However, an "empty" gun might not be (people get distracted, forget, or are generally careless), and taking someone's word for it without verifying for yourself can lead to just as bad - if not worse - of a situation.
ANY time you're handed a firearm, drop the magazine, rack the slide, check the chamber, open the cylinder, etc. If the owner gets pissed for you double checking, don't deal with them anymore. It might be the owner's own negligence that leads to a bad situation, but when the firearm is in your hands, it's your responsibility to make sure it doesn't go off except on the intended target.