From the Deutsche Bahn website:
The ticket is only transferable if it is not in the name and the trip – for tickets to
and return the outward journey – has not yet begun. Travelers with
they are registered entries
obliged, in the context of ticket inspection
On request to prove your identity through an official photo identification.
In English: tickets (all types) can not be assigned to a person. The Bahn Card 100 contains the name of the owner and, therefore, can not be transferred.
However, you can try to obtain a new card by convincing Deutsche Bahn that your friend has changed his name and address to his own. But I have no idea if that would work.