Taking life one step at a time...but it gets sticky
Honestly, it depends on my relationship with the friend. If me & the friend are close, yes, I would tell them that I saw their SO out with someone else. On the other hand, if we are not close, or are no longer friends, I might just mind my business. Not because I'm being mean, but because at that time it wouldn't be my place to tell them. My feelings for them or lack thereof are not invested. If me & the friend were close & I told them what I saw, for the fact that they would even think to call me a hater when I am looking out for their best interest would be silly. I'm trying to save them from heartbreak. The least they can do is take heed to what I am telling them & check for themselves to see if there is any merit to what I've told them.