Side dating. What would you do if you saw someone else?

[Anonymous]

What would you do if you saw a friend of yours (girl / guy, irrelevant), holding hands with someone else, not their partner? Would you confront them? Inform their partner? Or simply walk away? I think the answers would be different depending on how old you are.

[Avicenna]

I'd talk to them about it privately

[Nikki1989]

None of my business.