OK, my buddy just got a (mid 90s I think) Buick Park Avenue (that'* covered here, right?)
and the shifter works, but the pointer thing that tells what gear he'* in doesn't work, it just sits there
paid $100 for the car, and it is a great car, needs the cloth (?) roof replaced, and some small mechanical work, but I want to help him fix this, as it is very annoying

Has he got a column shift? Is the shift quadrant (the pointer thing) in the dash? I assume we're not talking about a console shifter here.

Typical GM setup for that was a spring clip attached to the lower edge of the shift bowl. A string (yes, really) led up from there to the pointer inside the little shift indicator assembly. The pointer was spring-loaded against the string: moving the shifter (and shift bowl) away from Park and into Drive (or whatever) would pull the pointer down the scale, and when moving the lever back into Park, the spring would pull the pointer back up again.

Your buddy'* car may have either a broken string or (more likely) a disconnected clip. You usually have to remove the clip from the edge of the shift bowl when servicing the column, and a lot of the time it'* forgotten when reassembling.

See if you can find light scratch marks in the paint around about the 4:00 position on the shift bowl, and a dangling clip somewhere nearby...