i would comment - and embarrass her - make her look a fool for expecting one trip with a metal detector to bring up a roman treasure, for being ungrateful, and also for being rather stupid in thinking you would not see the post
i'd give her your reason as to why you believe it to be a roman road - a link to an article or something - then ask her why she was upset about it?
ask if it was because she didn't find anything?
suggest that perhaps metal detecting is not the hobby for her if she was going to be so upset every time she didn't find anything - since it was going to happen a lot.
then say, so sorry you didn't find your fortune and you will not waste her time again by going out of your way to organise it for her.
because whatever she has said in public i bet she has said worse in private, so it lets all her friends know what an ungrateful fool she is
i don't agree with people whose auto response to any sort of conflict is to ignore it and say nothing - people need to be told when they are being out of order - or they just keep doing it.
ignoring someone does not 'get to' people more than saying something, they do not secretly admire you for your 'dignified silence', it just makes them think they have got away with it.