If I recall 18 is the standard rule of thumb for personality disorders because teens can by virtue of being teens display a myriad of behaviors that are actually part of their development. Brains aren't actually fully developed until 25, I believe. So these behaviors could be anything but.

I can sense your concern, however and respect such concern as noble.

Of course ups and downs are confusing. Hopefully through maturing alongside this friend you can hone a sense of your own expectations and boundaries as you proceed into adulthood.