Hi Izo,
This subject is very complex and there are others here who can do a better job of explaining the actual numbers (even this might not be enough to convince a die hard fanatic) but you might consider another line of argument. For me, it wouldn't matter if the numbers really did add up to 1914 because I'm still left with a big "So What" gap in my own mind. Much of what was so special about 1914 has been rendered meaningless by the passage of time. We don't have to argue and juggle numbers to prove that The Generation who saw "Christ made king" in 1914 are now all DEAD. Only tiny babies in that time hang on to life in their 90s now, hardley the fulfillment of a promise that "that generation" would not pass away before Armageddon.
Even the Society had to change the interpretation of the Generation to one that was so inclusive that it has lost all prophetic meaning since anyone born from then til infinity is now included giving us NO clear time limit on Armageddon's coming which was the only really important thing about 1914 in the first place. This alone should be enough to get any honest truth seeker to question all of the Watch Tower's other bogus "predictions". Without the power of "special God inspired" knowledge of Bible prophacy the Watch Tower Society is just another dooms day cult grasping at straws. Ask your friend, " Honestly, what is important about 1914 if it does not help us pinpoint Armageddon's start since Christ will still be our King either way?"