His unit may be too big and acting just like you describe. (Rule of thumb I always heard is about a ton per 600 sq.ft.) If his house is well insulated and doesn't have a lot of windows or sun exposure it will exacerbate the situation. If there is a humidifier for winter use simply shut off the water supply to it and see if it improves.
One other possibility is the unit is undercharged. This can cause icing of the evaporater coil and as it melts the humidity is blown right back into the dwelling.
Just a couple of guesses. The pros should be able to nail it right away.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".