The thing is here, if you were to tell that to a JW, they would say that quite possibly the WT did make a mistake. However, I'm sure they would jump on the fact that it drowned and quite possibly from the flood! Followed by ice being formed after the weather changed...It would fit the agenda.
Well, I suppose i would ask if i understand it correctly that the mammoth in question was drowned in rain, it then lay there a couple of weeks (this is how long they guess the decomposition took!) whily fluids from its corpse went into the ground around it (as described by those who found it) where the heaviest downpour in history didnt remove it, then, a couple of weeks into the flood, and supposedly beneath quite a lot of water, the corpse frooze solid, and remained frozen untill it was found?. I think i would ask why the mammoth didnt float away but remained in place, if they have an idea of the process that suddenly caused the temperature to be lowered locally around the mammoth, and i think i would ask why and where the WTC got its version of the story from (having material from creationists ready to show that that was the actual source).
The key is to keep recapping his version of the arguments, saying things like "okay, let me see if i understand you correctly now", repeat the silly thing he said, and then ask small questions to keep them thinking of it and dont change the subject (which they are very prone to! the key is to get them to think of the subject and not just argue with soundbites!). After a while the explanation will become so long and winded that they will realise:
a) the froozen mammoth isnt, perhaps, such a solid proof after all because the whole story about a mammoth suddenly freezing on the bottom of the ocean sounds kind of silly. b) how come this guy was able to poke gabing holes in such sure proof? why did i get in such a mess when the argument in the creation book sounded so convincing?
The point here is not to convince him in one go, but to make him do additional research on himself. Since the WT publish so little on the flood and evolution, that could be quite dangerous...
fun flood fact: who on the ark had syfilis, and was it the same person who had gonnoreah? (i know, it evolved/was created spontaniously after the flood, but its properly something they havent thought about before, and its allways interesting to ask if he feel that is a scientific explanation).