Depending on the circumstances, it could be. There is such a thing as third-party harassment, which occurs when someone is forced to view actions they find objectionable, even if they are not actually involved in the incident. Whether or not seeing two people kiss reaches that level, depends on factors you have not explained (and please, do NOT do so on a public board!)
However, regardless of whether it is or is not harassment, it is inappropriate for your friend and her co-worker to be kissing at work, and they can be disciplined for that regardless of what the person who saw them says.