A simple point is that in Matthew 24:36-51 are a series of parables.
Not, parable, parable, parable, prophecy, hypothetical situation...
So ask her:
1) Where is the evidence that Jesus intended this the be a prophecy?
2) What about the second faithful slave?
SECOND? Yes...the parables continue in Matthew 25. Read from 14-30.
14 “For it is just like a man about to travel abroad who summoned his slaves and entrusted his belongings to them. 15 He gave five talents * to one, two to another, and one to still another, to each according to his own ability, and he went abroad. 16 Immediately the one who received the five talents went and did business with them and gained five more. 17 Likewise, the one who received the two gained two more. 18 But the slave who received just one went off and dug in the ground and hid his master’s money. *
19 “After a long time, the master of those slaves came and settled accounts with them. + 20 So the one who had received the five talents came forward and brought five additional talents, saying, ‘Master, you entrusted five talents to me; see, I gained five talents more.’ + 21 His master said to him: ‘Well done, good and faithful slave! You were faithful over a few things. I will appoint you over many things. + Enter into the joy of your master.’ + 22 Next the one who had received the two talents came forward and said, ‘Master, you entrusted two talents to me; see, I gained two talents more.’ + 23 His master said to him: ‘Well done, good and faithful slave! You were faithful over a few things. I will appoint you over many things. Enter into the joy of your master.’
24 “Finally the slave who had received the one talent came forward and said: ‘Master, I knew you to be a demanding man, reaping where you did not sow and gathering where you did not winnow. + 25 So I grew afraid and went and hid your talent in the ground. Here, you have what is yours.’ 26 In reply his master said to him: ‘Wicked and sluggish slave, you knew, did you, that I reaped where I did not sow and gathered where I did not winnow? 27 Well, then, you should have deposited my money * with the bankers, and on my coming I would have received it back with interest.
28 “‘Therefore, take the talent away from him and give it to the one who has the ten talents. + 29 For to everyone who has, more will be given, and he will have an abundance. But the one who does not have, even what he has will be taken away from him. + 30 And throw the good-for-nothing slave out into the darkness outside. There is where his weeping and the gnashing of his teeth will be.’
So who is represented by the slave that received 5 talents, and who is represented by the slave that received 2 talents?
What? Can't she answer that?
Another great point is from the Watchtower 1981 Mar 1 pp.24-26. You can show her that from the WT CD-ROM. That's not apostate surely?
"Witnesses of Jehovah understand that the "slave" is comprised of all anointed Christians as a group on earth at any given time during the 19 centuries since Pentecost. Accordingly, the "domestics" are these followers of Christ as individuals.
Some readers may feel that this is a rather sectarian view of matters. Or they may object to the idea that the "slave" and the "domestics" represent the same class, one as a composite body and the other as individuals. The objectors may argue that not all of Christ's anointed disciples have a share in preparing the spiritual food, so that perhaps the "slave" pictures only the leading ones, and the "domestics" those they serve in the congregation.
There is no point in trying to force an interpretation of the parable. Self-deception is of no benefit and is spiritually damaging. Therefore, we must look to the Scriptures for an understanding. In doing this, what do we find?
[ ... ]
Thus we see a clear Scriptural basis for saying that all anointed followers of Christ Jesus make up God's "servant," with Jesus as its Master."
So in 1981, the idea that the leading ones alone (The Governing Body are surely the leading ones) are the FDS, was an idea of objectors? So how can it now be correct that the objectors had it right all along.
That and other good background points are in: http://www.jwfacts.com/watchtower/faithful-discreet-slave.php