It’s not the venues fault... it’s the clients fault for taking a chance knowing the uncertainties. Something tells me though that the venue never officially got booked, and you were given inaccurate info. They (the couple) probably were considering doing it there last month when there was speculation of indoor dining opening up on July 2nd, but when that never came to be, they probably decided to do a backyard party since the venue wasn’t an option anymore.
You are the one in fact not doing your homework, as you should’ve seen this coming and been on top of what the current regulations are and what your options would’ve been if the venue didn’t open.
I have 2 events that went from indoors to outdoors next month... did it affect me in any way... nope... why... bc we knew that this was a possibility and prepared for it ahead of time should the client choose that route.