2 Thes 2:3-4 says “the lawless one is revealed, the one doomed to perdition, who opposes and exalts himself above every so-called god and object of worship, so as to seat himself in the temple of God, claiming that he is a god” (NAB). The question is, does that refer to events that have already occurred, or will occur in the future? Haydock’s Commentary states:
By all these words is described to us the great antichrist, about the end of the world, according to the unexceptionable authority and consent of the ancient fathers… It may suffice to observe here that antichrist, the man of sin, the son of perdition, the wicked one, according to all the ancients, is to be one particular man, not so many different men. That he is to come a little while before the day of judgment. That he will make himself be adored, and pretend to be God
So, if the answer is “in the future”, then surely the apostasy mentioned in 2 Thes 2:3 is in the future too? Haydock suggest that the “apostasy” of which the passage speaks, is the apostasy of the antichrist himself (i.e. that the antichrist will be an apostate.) Also, while I’ve quoted a Catholic commentary here saying that this passage refers to the antichrist, all the Protestant commentaries I’ve looked at say the same thing. I’m not suggesting anyone should dwell excessively on these matters (people who spend too much time thinking or talking about the antichrist tend to go off in cultish directions), but the most sensible reading of this passage contradicts your LDS friend’s position that the apostasy of 2 Thes 2:3 has already happened.
The LDS doctrine of “Great Apostasy” is just taking Protestantism to its logical conclusion. Protestants believe that the Church at some undefined point went off in the wrong direction. LDS take this Protestant belief, and wind back the clock, siting this alleged apostasy earlier, even just slightly after the completion of the New Testament.