Sure. Sue the German government for not properly staffing immigration. They know exactly how many people are coming in and even have a decent idea of the number of passengers who will require secondary.

It's so typically FT that it's on the air carrier to play nanny to adult passengers who should evaluate the buying choices available to them and then act like adults and make a choice.

I don't see that EC 261/2004 applies and accordingly, the OP's friend is down to the COC which provide nothing.

What happens if there are no lines, but the passenger is pulled aside into secondary. What if it takes an hour to straighten things our and what if it is two days? Is EC 261/2004 really the means to deal with the reasonableness of government's immigration policies?

It's not clear whether the friend was rebooked and, if so, by whom or without fee. It does seem clear that there was an overnight and no hotel was provided.

Exact details would be helpful. To, from, nationality, was the line the only delay?