I disagree with most people here. I think your friend should have made some sort of effort. No one would have expected best bib and tucker, but as a sign of gratitude and respect to the elderly couple, who obviously viewed the occasion as an important one, then a change of clothes to suit the event and as a sign of respect and appreciation, would have been appropriate. Added to which, presumably she had taken at least some of her clothes off in order to have a half decent wash, so it would not have been difficult to put a fresh top on.