Hi Gabi,
 
silverface amps don't have datecodes on the tubecharts. There are a few other things to be taken note of:
- has the Fender-Logo a tail that underlines the whole thing? Then it must be between '68 and '74.
- has the grille cloth a thin aluminium frame? Then it's '68-'69
- has it a boost pull on volume pot? Then it's '78 or later
- The silverface Princeton was discontinued in 1979
All this can be found here:
http://www.ampwares.com/ffg/princeton_sf.html
The best way to find out the year are the transformer-codes. There should be three of them inside the chassis. The code consists of two lines. The second line is the important one. It starts with "EIA" and then 606. This number stands for the manufacturer - Schumacher in this case. Then there will be 3 or 4 digits left. The first one or two give you the year, the last two the week of the year it was made.
An example: The second line might read "EIA606-7843" - the transformer was made the 43rd week of 1978. If it reads "EIA606-051" it was made the 51st week of 1970. In the last case you have to guess the decade, which isn't difficult with the things I wrote above.
BTW: Silverface amps aren't much sought after YET - that might change in a few years. But the Princeton is nice little amp - so he should keep it and have fun with it once he dated it
All the best,
 
Vito.