How would you handle spoilt/stubborn teenagers?

That's a fantastic question!! If you get an answer, let me know, my teenage son is driving me crazy! He's bossy and arrogant and incredibly mean to his little sisters. I'm completely lost, nothing seems to be working. I've actually gotten him a therapist and brought in a home based therapy group to help me deal with him. Maybe your friend should consider something like that. If not to change him, then to help your friend deal with him. I avoided doing anything like that for a long time, but I just got to the end of my rope.

Grounding, added chores and taking away priveledges or cherished items until the kid decides to behave and cooperate. NEVER give in..go toe-to-toe until the kid respects discipline and a parent who IS one.
Stubborn kids may slam doors, yell, even hit..you have be calm, but hold to your guns. NO GUILT helps and you cannot let a teen manipulate you. You are the parent and they can get friends elsewhere. Know who they hang out with and watch them like a hawk. OIf you do not approve of the friends, say NO WAY! Make them get into a better set of activities..sports or music, etc...and make new friends.
They will hate your friend now..but love their parent later when they have a better life.
Parenting is NOT for the weak!