Can a friend be detained for riding a motorcycle and hitting someone?

It's possible. I don't remember the specific road traffic law, which says that you can be detained for less than 15 days or fined 200-2000. The fine or detention is decided by the police. The figure of the fine seems to be inaccurate. You'd better check. In this case, generally speaking, the police don't really want to detain your friend. They may want to get some benefits, or they may want to scare him into making positive compensation. Mainly to deal with the elderly, it is inevitable for the police to do something.