So clearly there is some debate here as to what the phrase "All-Time High" actually means. Perhaps in a day or two this will all be irrelevant but for the time being, what do you consider to be the most accurate measure of bitcoin's all-time high? Has LFC won the bet yet, or nah?
For any practical purpose, the sides should agree beforehand on which exchange or index to use for tracking the price - there's no other way around it. If it's a casual discussion about ATH, then a few percents of difference between exchanges can be ignored.
I think this particular bet is almost decided, because the chance that the price will establish a new ATH on all exchanges in this year or the next one is extremely high. But without an established agreement on where to track the price, this bet can't be cleanly resolved until overwhelming majority of exchanges show a new ATH.