Would this be legal?
Probably at this scale. Not necessarily extrapolated to a large commercial operation involving more parties.
At some point it becomes a payment system and a financial enterprise that becomes subject to financial regulation.
Could her transfer be viewed as a donation as well?
This is not a donation transaction and efforts to characterize ...
With a legal contract in both countries,
listing the purpose (transfer of funds) and accounts being used ;
togeather with a tax consultant (Steuerberater) protocoling the money transfers ;
who would satisfy queries from the banks (that it is not money laundering) and tax authorities (that it is not income)
would probably be doable since it would then be ...
can this be treated as [being] money laundering?
People can treat it however they want. The facts you present do not constitute money laundering, but they likely would give rise to the suspicion of laundering.