I've been in barns that blanket when the temps dip below 60 degrees, and other barns that don't blanket until the snow falls. Within each barn, there is some variation depending on the type of horse (my arab NEVER wanted a blanket; my OTTB ALWAYS wants one) and the type of owner (my horse is a horse vs my horse is my baby). As a barn owner, you probably know the owner fairly well, and the horse fairly well. Would it have been possible to call the owner and ask about blanketing? Does this owner NEVER blanket? In this particular case, I think you took action and have to take responsibility for the action (i.e., the loss), but going forward, you might want to post general temp/blanketing guidelines and chat with the owners about their blanketing plans. If they start depending on you to blanket, then you're going to be stuck doing it daily. The owner may -- at some point -- make this up to you. It might just take him/her a little while to figure out the right thing to do.
You're a good barn owner. The good karma will flow your way.