The OPs friend has AC so a twig wouldn't get past the evaporator to sabotage the blend door but something could have fallen down one of the vents (especially the vent at the top of the dash). I know for sure I've got a nut, a screw and a rubber grommet fall down there - just lucky they haven't messed with the blend door yet.
Pulling the dash is a lot of work just to get some junk out of the heater box.
If your friends 98 still has the original heater core it may be a good time to swap that out to make a big job worthwhile.
To truly isolate the heater core, an isolation valve would be needed on each heater hose. I know it's supposed to flow in one direction but if you put valve on one hose, there's nothing to stop hot coolant running back up the other hose into the heater core.
Or run a bypass hose if you don't need the heater at all.
The inlet and outlet hoses are different sizes - 5/8 and 11/16 - but I'm not sure which is which - both my hoses are marked 'IN'
I ran a short length of 3/4" hose from the thermostat to a brass connector (red arrow), then another length of 5/8" hose around the air intake to the other heater hose connector tube (yellow arrow) which is attached the to the water pump housing.
I probably didn't need to run the hose around the air intake - was just trying to avoid kinks in the hose.
EDIT: The original heater hoses are in the pic, just not connected.