Mmm, a good question. I'm afraid your friend might have wriggled out of this one.

For the bet to be paid, both words would have to be misused in 15 posts. I suspect there are no posts that satisfy that. Sorry.

Send him this link: then/than.

There is one small hope for you. In Europe, they go for the spirit of the wording in law and not the exact meaning (Latin law) as we do in the UK and US. If one of you is European, you can claim the spirit of the criteria was known to you both - which I suspect it was. However, that's your only hope. The words mean that an incident of "then" and an incident of "than" is required in a post for it to count.