If you are "asking" then it means you think a line may have been crossed. Would the friend kiss you the same way if your significant other were standing beside you? If someone showed your significant other a photo or video clip of the kiss taking place would your mate think it's not big deal? Would it bother you if your mate was being kissed by their friend?
Cheating like beauty is in the eye of the beholder. Some people it's possible to "emotionally cheat" (without ever kissing), others view flirting as cheating, another may consider spending time with secret friends (meetings, lunches, dinners, or attending activities....etc) is a form of cheating. Knowing your partner is a major key in determining if a line has been crossed in "their minds". Also knowing how you would feel if things were the other way around helps one stay on track.