I put the OP's friend's travel details into TIMATIC and got the following:
Type: Notice
Germany - Transit Passport
Important:
Passports must be signed by the holder, unless:
1. The issuing country permits a fingerprint instead of a signature; or
2. The passport contains a remark stating that a signature is not required or that the holder cannot sign the passport.
I'll let you folks argue the absurdity of the situation.