As someone who is informed on the matter at hand, let me explain.
The Church before Vatican II (and still today) requires that all Catholic abstain from meat on ALL Fridays (NOT only Lent), except if the Friday fall upon a solemnity (first-class feast, e.g. the Friday in Easter Week, St. Joseph’s Feast Day, Holy days, Epiphany, etc.). Now, before Vatican II (not really Vatican II, but prior to the new Canon Law 1983), there were NO dispensations or exceptions. In the new canon law, the Bishops are permitted to allow other foods to be substituted for meat, and now even are permitted (at least in the US, but I think that it is only for certain countries, not a universal norm) to substitute “other works” (e.g. making the Stations instead of doing abstenance). Now, the universal norm of the Church (and what the opinion of one of the three canon lawyers on the tribunal in my Archdiocese of Atlanta–maybe more but at least one) is that all Catholics are required to abstain from meat still. Now, as I said, there are some dispensations, but these are not universal, and only the Bishops conference can name such exceptions.
In any event, the mortal sin was not in the act itself. The mortal sin was in disobedience. For example, before 1953, if one were going to receive Holy Communion, he could not eat nor drink ANYTHING (including water) after midnight. Now, if you lived before that time, if you had a sip of water at 12:00 and 1 second, and then you received Holy Communion that morning (considering that you knew it was after midnight), then it would be a MORTAL SIN and a sacrilege, just as today if you have a bite of a sandwich, and then you receive Communion 59 minutes and 59 seconds later, that it is a mortal sin (if you do so willingly, and knowing that it had not been a full hour). The mortal sin is in the disobedience, not in the act itself. That is why, if the Bishops approve an exception (e.g. if they say you can give up chocolate instead of meat on Friday), then if you follow the exception, you are not committing a mortal sin, since you are being obedient.
Now, as far as his question about those who committed the sin before: since they committed the sin, which is a sin of disobedience and not the act itself, it is still a mortal sin. It would be like saying: is it still a sin for a Jew to eat shrimp if he did it the day before Christ proclaimed that all food is clean? Yes, because he was being disobedient to the judicial rules of the Old Law. But that sort of sin is an act of disobedience to the Old Law, not a sin in the act itself.
As a side note, my personal opinion that all the dispensations and exceptions are quite ridiculous. Not only can you pretty much do anything (even maybe a decade of the Rosary, depending upon what your Bishops may say–and who knows these days what they will say) for your “abstenance” on Friday. Moreover, it makes much more sense to have a catholic (that is universal) law for obedience for this and many other things in the canon law, as well as in the Mass. The more uniform and hence catholic things are, the more the CATHOLIC Church is seen to be what She is: Catholic. I suppose it does not much matter since nearly all those who know of the rules of abstenance (and care about followig the rules of the Church) follow the meat abstenance, while those who chose to be disobedient or are simply ignorant do nothing. I would assume only the most staund neo-cons (or those who really don’t want to gie something up but just want to do something easy or easier) would actually follow the dispensations.
N.B. Canons from canon law related are: 1249-1253 (BK IV, II, ii, 2)