Actually, what the hell.
This is pretty rough. The odds according to anydice.com of rolling an 18 is 1.62%.
That checks. He rolled 4 18's and 2 15s. I won't try to calculate the odds of getting exactly a 15, but on a 15 or better.
The odds of rolling a 15 or greater is about 23.15%.
Just as we know the odds of a 18 on 3d6 is 1/128 (6x6x6) we could figure out the odds of 4x18 + 2x15 in any order would probably be to the tune of 1.62/100 * 1.62/100 * 1.62/100 * 1.62/100 * 23.15/100 * 23.15/100
Hell, the odds are getting 4 18's with four rolls is about 0.000007%, let alone the odds of the two 15's or greater. We'd have to change the odds to reflect he had 6 rolls, not four. Odds of him being full of shit? about 100%
Also, interested in the Minotuar vs. 1st level Thief. Rules are pretty clear on this. Max thief HPs at 1st level = 8, attacks 1, damage 1-6 (+2 for strength) (5.5 damage per attack.) Thief thaco - 20 (to hit Minotaur: 12). Max Thief AC at 1st level: 3. Minotaur: AC 6, THACO 13 (to hit Thief, 10), 2x attacks per round. Damage per attack - 2-8, HPs 6d8's +3 (or 4.5 hps per die = 30 hps.)
Ok, so the thief would need to hit with a 14 or better every round and hit about 6 times, so a 35% chance six times, or 0.1838%. The Minotaur would need to roll a 10 or better and hit twice (55% chance of hitting.) So, the Minotaur would need to miss 12 times in a row, which is about 0.0069% chance.