Helping Your Adult Kid

The topic of dysfunctional young adult behavior is brought up in my office every day by loving and attentive parents (exhausted too).
I have broken the overall topic into three concerning behavioral types:
Failing to launch
Mental Illness
In a simple format, understanding that I cannot and am not diagnosing someone that I have not personally met with, let me break down these three distinct challenges.

Failing to launch/Failing to Thrive: An under-earning, underachieving young adult. Perhaps a high school or college graduate that is working only part-time, being financially supported by parents or relatives, although he/she is intelligent, able-bodied, sound, and surrounded by nice people. What is going on? Why can't she/he get a job? Working in a pizza shop 25 hours a week at the age of 24, while mom and dad subsidize this lazy lifestyle is not thriving...
a) Who is paying for him/her? This includes their total cost of living, minus rent/shelter: car payment, health insurance, gas, money for fast food, phone bill. If this is you, the parent: STOP. If you must, get them a flip phone (no cell data!), remove your HULU and Netflix, stop bringing home delicious fast food and offering to rescue them. DO NOT provide an emergency credit card either. How emergent can it be while living at home with a well-running vehicle, AAA, unlimited cell data and all expenses paid? 
Do not do for your child what he can do for himself.
Make the nest very uncomfortable. Why else would anyone leave a cushy paid-for lifestyle? 

Mental Illness: This is serious work, navigating a psychiatric condition with a loved one. Usually the family of a loved one living with a metal illness has spent years trying to determine the underlying causes and conditions that are preventing someone from living life to the fullest. But, please remember that  psychiatric diagnosis (ADHD, PTSD, OCD, Depression, etc,) are never reasons to lash out, assault others verbally or physically, or blame others for one's predicament in life. 
It's often hard to know when to tolerate avoidant behavior ("He's too nervous to apply," "She has panic attacks when she drives,") - my rule of thumb is this: If you are refusing help, blaming others, and taking no positive action, there is very little familial compassion left for someone to continue to falter/stumble. At this point, the family/parents would benefit more from coaching and counseling than the young adult himself. Setting healthy boundaries and limits with professional guidance is the best beginning; job coaches can provide appropriate career goals and thankfully, support groups exist for every condition. There is something for everyone and we should encourage those with psychological instability to be as self-supporting as possible.

Addiction: This may be the hardest behavior to identify, as most addictions are kept secretive and the addict will lie to cover up his/her actions, after all, it's usually hard to prove! Furthermore, families are initially reluctant to admit that their loved one is engaging in such self-destructive behavior.  
Signs of addiction including: Lying, stealing, arrests, mysterious and unexplained behavior, long and dramatic hard-to-believe stories, job absence, extended illness, poor physical appearance, failure to complete anything, multiple failed relationships, and finally, blaming others. 
This family needs to attend an Alanon meeting (free!) to better understand the tenous nature of loving and caring for the addict.

Blurred Lines: There are often overlaps between these three; we cannot accurately diagnose a severe mental health condition while someone is in active addiction, and we know that addicts will attempt to regulate their dysregulated mood with drugs (alcohol, weeds, pills). Life is a series of dominoes, right? If I feel badly about my future, I will work less efficiently, which leads to more stress and less success. How does one eat an elephant? One bite at a time.