So far, what you have described sounds like a DWM Alphabet commercial Luger.
These were made between WW-I and WW-II from about 1920 through 1930. If there is a suffix letter under the serial number digits on the front of the frame above the trigger guard, that is part of the serial number and will help get closer to it's actual manufacture period.
These are very well made pistols. They don't have as much collector value as military guns, but (of course) this one is priceless to your friend...
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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