Nick Reiner's Mental Illness, Drug Abuse Were Unsolvable Puzzle, Expert Says

Nick Reiner was an unsolvable dilemma for his parents ... so says a substance abuse treatment expert.

Nick Mathews -- the founder and CEO of Southern California-based treatment center Stillwater Behavioral Health -- joined us on "TMZ Live," and we asked him about Rob Reiner and Michele Singer Reiner's son, who is charged with their murders.

For Mathews, Nick presented a treatment provider's worst nightmare ... he was diagnosed with schizophrenia, but had been in and out of rehab for years and struggled with substance abuse.

Just listen to Mathews explain all the challenges Nick would present for the rehab centers and doctors tasked with treating him ... there's no quick, easy fix or even a cut-and-dry solution here.

Plus, Nick is an adult ... and Rob and Michele couldn't make him stay in rehab centers long enough to get the treatment he clearly needed, Mathews says.

As Mathews explains, there was a perfect storm brewing here ... and now Rob and Michele are dead, and their son sounds like he's planning to plead not guilty by reason of insanity.