I've never heard of it either. You might try Flixbus?
https://www.flixbus.com/bus-routes/bus-naples-sorrento
Understand that a long-distance bus isn't going to get them to their hotel in Sorrento so depending on where that is, they may need additional transport from the drop-off point. As well, look at the Campania Express train (seasonal) and Circumvesuviana commuter train.
While the Circumvesuviana is a battered thing lacking in air conditioning and other creature comforts, it's the one option that will provide the most daily runs between Naples and Sorrento to choose from. Otherwise, they'd could waste a lot of time sitting around in Naples before the next train/bus. They would need to keep a tight grip on their valuables on the train, and would have the same issue getting from station>hotel if it's not within easy walking distance.
As you're doing the booking, there may be concern that they can fend for themselves, with their luggage, as far as finding the bus station, or finding the Garibaldi train station (where the Circumvesuviana and Campania Express come and go from; is connected to Centrale), buying their tickets and finding their train? Honestly, if they aren't up to any of this, I'd spend the money and book them a station-to-hotel private transfer.
https://www.sorrentoinsider.com/en/naples-to-sorrento-train-schedule
https://www.sorrentoinsider.com/en/campania-express-train-schedule