Notice to a sub tenant (untermieter) (Munich)

Can anyone help with some advice please?
My friend issued her sub tenant, previously her partner, with 3 months written notice which is now up. Unfortunately he says that he isn't ready to leave yet, and believes the notice to be illegal. Does anyone know how this situation might be resolved please? Will my friend have to employ a lawyer? Does she have to wait until he is ready? Can she just change the locks? I'm grateful for any thoughts. Thank you.