Should the solar fluid be drained during winter? Frost damage?

Good day. Our neighbor needs to repair his solar piping in the attic. It is leaking. Now he wants to drain the solar fluid and only refill it again in spring. The oil heating system is designed so that the solar system is not absolutely necessary. (It was installed afterwards). Question: -Does he have to expect frost damage to the collectors if he drains the solar fluid and leaves the system as it is over the winter? The pump, etc., will of course be switched off. Or can he do this as an exception. Thank you very much

What reason should he have for leaving it empty other than to postpone the repair appointment? I would not want to keep it empty and leaking over the winter. Without more detailed technical consideration of how the filling behaves under oxygen in a stationary state, I would be concerned that drying out without cleaning treatment could at least damage the pump.

Definitely fill it correctly. In the end, you'll have rust or condensation inside. Condensation -> frost damage, if things go badly.