To try and summarise so far...
What most likely happened to the Piper is that it iced up. At 5,000 ft, in a front with 7 C on the deck, it would have been flying with an OAT of about -5 C, peak icing. A request to descend to 2,300 ft would fit with that.
Under EASA regs single engine single pilot commercial ops at night or IMC are limited to turbines. As this was a SEP Malibu, the flight could not have been legal if what was in any sense legal. The pilot didn't have a full IR rating; if had have had one, the flight would have been safely conducted in Class A above FL150, above the icing layer. SEP over water isn't dangerous.
The only ways this flight could have been conducted legally would be if the cost was being shared, in this case 50/50 pilot/passenger, or if the pilot was not being paid at all. I don't think someone with a CCJ against them would have been able to afford the former, but that doesn't preclude the latter. Indeed I wouldn't be surprised if someone hired the Piper from the owner for the cost of the flight (fuel, fees etc), but the pilot was purely volunteering. That he was not qualified to undertake the flight safely (i.e. IFR, high in the airways) but did it anyway is poor airmanship. Yes he could have flown the whole trip below the freezing level, about 3,000 ft, but that would have been... bold.