OP says that the user made only one bet and won, so how exactly does this turn into arbitrage betting? I am still trying to understand.
@badpeanut, you will probably have to try Casinoguru or askgamblers where they will be compelled to provide proof that shows that the user actually did arbitrage betting. Without much proof of screenshots from any of the parties, it's had for us to know what made them accuse your friend or if they just made it up.