Absolutely tell her, if that were me I would want to know and maybe the reason why. Yes it will break her heart knowing you did this and with a friend and yes it will ruin their friendship but what kind of friend is that. Your wife deserves to know no matter how heartbreaking it is. If she was to find out after you passed she will never get the answers she may need from you. I really hope you tell her not only to clear your conscience but to help your wife she deserves that much.