Actually, troubleshooting the AFCS on the helideck was a sensible course of action if the crew suspected an AP problem. As all S76 pilots would know, the test is simple. Whilst on the ground just turn off both AP and press the TEST button on the autopilot controller. This Level 1 test checks the hardware and software considered to be flight critical. LVL-1 PASS or FAIL is shown on the AL-300 on completion. LVL-1 PASS and you're good to go, obviously (the test function is disabled by AOG logic which is why you need to do it on the ground). Any system malfunctions will be indicated with a code that you can refer to in the RFM for more detailed explanation.
If LVL-1 test FAIL whilst offshore, it would be sensible to shut down and seek advice from maintenance. I suspect that advice would be to disembark the passengers and ferry it back to the maintenance base (and there would be paperwork necessary for that to happen I would expect).