Can you be friends with an ex?

After having my friend run out of the club on NYE because he saw his ex near her ex, I've begun debating, is it ok to be friends with an ex?

• Of course you can
  • 
  • 
  • 
  • 
  • 
• Absolutely not
  • 
  • 
  • 
  • 
  • 
• Depends on the ex
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  • 
  •