This is an easy question to answer.
1) If the house has no trouble at all staying below 50% humidity and keep fairly stable, then no they don't need a dehumidifier.
2) If the house is going to swing the humidity a lot or they struggle to get it below 50%, then absolutely yes a dehumidifier.
Now, unrelated to cabinets, if they can't keep their house below 50% then the best money spent isn't a dehumidifier but rather finding out where the humidity is coming from and fix that. Leaky ductwork, oversized HVAC units, dirty coils, bad envelope sealing, poor bathroom ventilation, huge-ass aquariums.
If I were you I wouldn't be talking to them regarding whether they need a new machine (dehumidifier), that's the HVAC guy's job. You need to be clear to them that they need to keep the humidity 40-50% stable. Let them decide how that needs to happen.
For what it's worth we used hard maple on nearly all our paint grade jobs for various reasons. We seldom have more than a little caulk here and there that we have to deal with regarding wood movement. Maple isn't exactly one of the worst offenders here.