
Pizza Lord |
The possessor can remain in possession of the person-rock. They likely couldn't see or hear or know what's going on around them (unless they have other senses that function while they're possessing something). Or they can leave. Or they can hang around a little while and then leave, but I'd say after a minute or so it starts to get hard to know just how much time has actually passed without some internal clock ability or just strictly focusing on mentally counting the seconds.
Just because an ongoing spell's target become an invalid target doesn't mean the spell or its effect ends. In most cases it just becomes moot or irrelevant. It's still there. If you charm person a humanoid and they somehow become a different creature (including type), they're still charmed. A protection from fire spell cast on a person who gets turned to stone would still have the spell's protection on their statue until the duration expired (and also have a stone object's hardness in addition if hit with fire damage), even though you can't normally cast protection from fire on a statue. Similarly, they'd still have a bear's endurance spell active, but it would be moot, since a statue has no Con score, but if they were returned to flesh before the duration expired, they'd still have the benefits.