I'm sure there are other x-catholics who had the same understanding my wife had, it was a sin, and had to be be confessed to the priest if they ate meat on any Friday.
Agreed. There are millions of Catholic with little to no understanding of the things of their faith. This is to our shame and is a reason why your wife is blessed to have met you - she may never have "met" Christ in a real way within the Church but now is now bound for Heaven (unless I miss my guess). Despite all of my serious disagreements with much of Protestantism. I am infinitely more pleased with a "cradle Catholic" getting saved there than staying here in ignorance (not the same thing as calling her ignorant).
That having been said. If you had decided (perhaps with a group of other Christians), to observe two days of prayer and fasting (Scriptural, you agree?) - say over the financing of a new church building project or a missions project - and you then decided you were too hungry to continue so you snuck a cheeseburger. Would you not feel you had sinned?