It's in your interest, I would say, to inform the landlord. You are perfectly within your right to sublet a room usually but this has to be done with the consent of the landlord / management company as doing it otherwise would be doing so against your contractual terms with the landlord

Besides your friend will presumably register at your apartment in the meantime and receive post to your address. It's a lot simpler to inform the landlord and do everything properly. It's not very complicated, sublet contract can be obtained from mieterverband.ch (let me know if you can't find it) and you just need to send an email / letter with a copy of the subletting contract and it's all done.