Damp rids are ok in closets. I dont believe they'd completely remedy OPs legitimate concerns.
He's really not venting it outside, he's venting it towards holes that lead to outside. If his soffit holes and ridgevent are not working together, all that humidity during winter months may be a problem if its not immediately swept away by air/wind movement.
People use damp rid in the crawl spaces all the time, for the same reason he'd need to use them in the attic. Moisture/condensation buildup due to heat/cold differentials.
Its not a complete fix in and of itself, but if his only option is to vent into the attic, and he cant install a vent to connect his ducting to, then the damp rid will surely help remediate localized condensation/mold.
Edit: Dropped my phone and hit post. Anyway, a lot of it will have to do with like you said the exchange in the attic, the attic size, the conditions of the air hes pumping in up there, etc. But if he can't do anything the change his current setup, id suggest just adding as much dessicant as needed to keep the attic from turning into mold city