On the website of Deutsche Bahn:
The ticket is transferable only if it does not appear in the name and travel – for tickets of
and return the trip to go – have not started yet. Travelers with
are registered notes
obliged, as part of the control of the notes
upon request, to prove their identity by means of an official photo ID.
In English: tickets (all kinds) can not be attributed to a person. The Bahn Card 100 contains the name of the owner and can not be transferred.
However, you can try to get a new card by convincing Deutsche Bahn that your friend has changed his name and address for yours. But I do not know if it would work.