The GVB travel planner suggests that it's about a 40 minute journey (including walking & waiting) by any of these routes:
1. 17 minutes on the train to Centraal station and walk (18 minutes) to Dam ;
2. 17 minutes on the train to Centraal station. A short walk to tramstop 'CS Tram Oostzijde'. 3 minutes on Tram 24
3. 8 minutes on the train to Zuid station. Then Tram 5 (20 minutes) to tramstop 'Dam/Paleisstraat'.
There are also other routes available:
http://journeyplanner.9292.nl/Default.aspx
However that 18 minute estimate, for the walk from Centraal station to Dam, seems excessive to me. Unless your mate will have lots of stuff to carry, that's probably the best way to go.
Chris