If kept in good shape, it's value will only go up due to Marlin being purchased by the same peanut counting company that also owns Remington. The boys on the Marlin web site are already complaining about the poor quality that is now coming out under the marlin name since the purchase of that company.
I have to agree with Reddog about the Marlin 39 being one of the best, and maybe best lever 22 made. A lot of Marlin rifle fans feel that way and that is one reason the 39 commands such a high resale price if in good condition.
What it is worth today, may not be the case one year from now. I think it will go up higher very fast for the older Marlins 39s if their quality control does go down like it appears to be doing.
Tell your friend to enjoy shooting it, but take care of it and do not abuse it and it will return a good investment if he should have to ever sell it down the road.
RELH