I really can't tell you how the second officers/relief pilots are exactly handled at KLM since I am not flying for them.
In my airline relief pilots have a complete typerating with the restriction of not being allowed to perform takeoffs and landings. They are allowed to occupy the right seat above 20.000'. I guess it is similar in other airlines.
As for the requirements... I guess that depends on the company. At our outfit most of the relief pilots are former Flight Engineers which - when the last aircraft requiring a FE left the company - where either too old or did not want to become a FO, or they did not pass the selection process for a FO position (they had to do the same selection as required for someone trying to get to the academy or for Ready Entry Pilots).
Some airlines, e.g. KLM or (as far as I know) Cathay have Second Officer programms in which the SO flys as relief pilot for the first couple of years. As for the details about that, you'd better be off asking someone flying there - for example your friend.