Funny - I've probably sold 5 bikes in the last year, and I think for 4 out of the 5, I had calls or emails where the first question the person asked me was the weight limits of the bike.
Heavy people are generally only too painfully aware of their weight, and the limits it places on them as to being able to be comfortable on an airplane or to be able to sit on certain kinds of delicate furniture and machines like bikes. They've undoubtedly been embarrassed by breaking something at some point.
I almost gotta think that lady bought the bike for someone else. She didn't ask to test ride it. She probably didn't intend to ride it.
Myself, before I sold somebody a bike that was gonna break under them, waste their money and maybe even hurt them, I would absolutely be compelled to mention that it was built to certain specs. You know? You don't allow a person who doesn't know what they are doing to handle a loaded gun. If I'm selling somebody something that's not going to work, I tell them that up front.
In the case of a very fat lady, I might say something like, "Ma'm, a gentleman never comments on certain things about a lady - her age, and so on, and I would never want to offend you or hurt your feelings - but even more, I would not want to take your money if you can't use the bike, or to allow you hurt yourself because I did not know how to say this politely. Just let me warn you not to haul anything heavy on this trike, because it is designed with a combined rider and cargo weight limit of..." and here is where you simply LIE and say "...about 175 pounds."
That allows you to underestimate her weight. At that point, if she laughs and says, "Heck, I weight twice that much," you can say, "I never would have guessed."
If a speech like that offends someone, then that's not your fault. You express your concern for their safety, and then state the weight limit lower than it really is if you are afraid to suggest that they might actually weigh more than 250 pounds.