I heard that you need a biometric passport if you want to enter USA. However, my friend has a biometric passport, but no fingerprints in it. Is this enough? And how long has the passport to be valid? Is it okay if it expires on the day after his return date?
What kind of passport do I need?
See the passport requirements for travel under the Visa Waiver Program. (The VWP applies to German citizens among others.) It says this about biometric passports:
October 26, 2006 – Each Visa Waiver Program passport issued on or after this date must be an electronic passport with a digital chip containing biometric information about the passport owner.
My passport doesn't contain fingerprints (to my knowledge), but that wasn't a problem (2011). What happens is they take your fingerprints & photograph when entering the country. (If the passport does contain your fingerprint data already, I'm not sure if they'd just read that or take the fingerprints again.)
So yes, your friend should be fine.
To be more precise, the passport only needs to be electronic & biometric if it was issued on or after October 26, 2006. (Except for citizens of Czech Republic, Estonia, Slovakia, Hungary, Latvia, Lithuania, Republic of Malta, South Korea, and Greece who always need an electronic passport.) In any case it needs to be machine readable (i.e. contain two lines of text with numbers and chevrons (<<<) at the bottom).
How long does the passport have to be valid?
To answer your updated question:
And how long has the passport to be valid? Is it okay if it expires on the day after his return date?
For citizens of Germany (and most other ESTA-eligible countries), yes, this is okay; no extra validity is required. The passport only needs to be valid for the duration of the visit.
Visitors traveling to the United States are required to be in possession of passports that are valid for six months beyond the period of their intended stay in the United States. Citizens of the countries listed below are exempt the six-month rule and need only have a passport valid for their intended period of stay.
Germany is on the (long) list of counties that follows.
Finally, tell your friend to just go ahead with the ESTA application already (unless the trip is a very long time away; the authorisation is valid for multiple trips over a period of two years). As someone wrote on the internets:
Have you signed up for the ESTA program yet...if not you need to do it right away. Getting your Esta clearance is another confirmation of your passport being fine.