rent a car themselves or
Yes. Every rental website will ask for your age and in most cases they will give you car. They may charge you extra for it, but that's it. I had no problem finding cars for a 23 year old on Avis (which was the only one I tried)
drive a car, that a friend of the age 25 or older rented
Any additional driver will have to be registered and almost all rental companies charge extra for a second driver. That charge may depend on the age of the driver. Put your info in the website and see what you get.
Furthermore, would I need an international driver's license for renting and/or driving or would the german one suffice?
The german one is fine.
to get around the surcharges?
You can not get around the surcharge since all driver must be listed and paid for. Keep in mind the official renter is typically the ONLY person that is allowed to pick and drop off the car, so the 25-year old has to be show up in person (with their credit card).
... what would that mean regarding insurance of the car for the duration of the rental?
Insurance is good topic to research carefully. It's one of the biggest money makers for rental car companies and they try to sneak it in at the last minute at extortionary prices. This can easily double your rental price ! Rules around this are quite complicated and I recommend reading terms and conditions very carefully. Often you are covered through your primary insurance in Germany (if you have one) and/or through the credit card but that depends a lot on the details.