I'm not a legal expert but I can't see how you could be held responsible...the family CHOSE to bring him to their private home -- where they know full well they don't have a nursing staff. My parents/their siblings used to bring my grandparents to family gatherings on a weekly basis, and the only time they involved LTC staff was if they felt my grandparent needed to go back earlier than expected, they called to give the nurse a heads up.
As for leaving the facility to assess him, there is no way in hades I would be comfortable with that. What about the res's in your facility that you ARE responsible for?? I believe the term the BON uses for this is "abandonment."
And anyway, nurses are NOT trained as first responders! You have no business going into someone else's home and deciding what to do.... even in your facility, if someone falls and sustains an injury you call 911 while monitoring the pt's CABs. In this case, appropriate action is for his wife to call 911, and the dispatcher walking her through monitoring his CABs until EMS arrives.
I probably would have written an incident report that included the info from the call from Life Alert, and that the family had been instructed to call 911 in case of emergency -- IF you had given that instruction yourself (either in person or over the phone.) I would think it appropriate to give the ADON a heads up too, as 1) risk management might need to get involved and 2) if injuries happened, that would impact his nursing care upon his return to the facility.
But actually *handling* the fall situation, or documenting the actions taken outside of your facility (e.g. immediate post-fall assessment, or res's words to EMS) -- absolutely not.