Is there a setting that determines the PM limit, and can it be changed for one user?
The limit setting is found in the Group Management. Next to the group that the user is in click 'Site'. In the first column, 5th line down, you'll see 'Total # of private topics allowed'.
I think that the only way that you can increase it for that user (or selected users) would be to create another group, transfer him / her to that group and increase the limit.
