A friend kiss?

So one of my friend has a boyfriend which she's serious about, but gave this one guy a kiss on the cheek. It wasn't super long and it's not like she put a ton of thought in it; the guy she kissed was a close friend and it was among mutual friends so it didn't seem like a big deal. Are these kind of kisses okay when your invested in a relationship with someone else? Tell me what you think and why please :)

Basically, this is your friend's business, not yours. All you need to know is what's ok for you and what concords with whatever agreements you and your significant other have agreed upon. What other people do isn't your concern.

A kiss is an emotional expression. You have to consider why she did it. If the kiss on his cheek was a clearly understood response to something he did for her, that's different than doing it for no reason. For example if a mom kisses a rescue worker that just saved her child, nobody could see anything wrong with that. I've received a number of thank you kisses when I would have much preferred a handshake. :-)