Roof leak, backed up plumbing, dangerous situation like broken window or exposed electrical....it gets fixed asap.  

The market is the market, but people are always going to need a roof over their heads, so there will always be demand.  

Depends on the foundation, plumbing, electrical and roof.

This time through I saw that he had completed a lot more of the work and had gotten a new roof.

Empty can be good for you if you need capital improvements that are inconvenient for a tenant like parking, roofing or electrical. 

Such as missing roofing tiles, sagging ceiling, cracks in the foundation etc.  

I have done repairs for other investors due to home inspection things as simple as adding an attic fan, to repairing roof leaks that were obviously there way before the rehab, to ripping out an entire bathroom, and starting from new.  

Not to rough inside but it does need a new roof  and gutters ASAP and Central heat and air installed.  

and i havent ruled out contacting Los Angeles Child Services because in a way, now that i have been told by the mother's mother than there was fraud involved in regaining custody, doesn't 'see something, say something' apply especially when its going on under one's own roof?

The roof was partially gone, so it flooded during the next rain.