These answers are kind of dancing around the question....
The answer is : really fucking rare. less than 1/10,000 people, probably. When I say this, I'm assuming a bench press that would be passable in a powerlifting meet. No bro-spots, no heaving/bouncing off the chest.
I'll give you a reference point to work with. You can find folks who don't work out but can still bench 300. It's uncommon, but it happens. You will never, ever find a non-lifter who is capable of a 500 bench. The number of living humans who are under 200 lbs and can do it is almost certainly less than 20. You'll likely never see a 500 lbs bench presser in a commercial gym. It's an extremely rare thing. I have never seen a 500 lbs bench press in a commercial gym. Even in a hardcore gym, like the Metroflex gym I go to, you will likely only have a few members who can do it. At my gym, I'm fairly certain the number is less than 5.
So in summary and to answer your question directly, just being a 'big guy' does not mean a person is likely to have accomplished this feat. I'm not saying the person you're talking about is lying, by any means, but it is a distinct possibility.