On my first trip to Latin America I discovered the custom of greeting with a kiss. As a hot blooded 18 year old I was pretty happy when a girl I liked was going to greet me with a kiss.

Just when I was about to get my kiss my brother in-law interrupted us!

He then (quoting Romans 16:16 no doubt) said to me in English, “Let it be a holy kiss my brother.”
Holy, in is very essence, means to use something for a special purpose. So if we are married a passionate kiss is holy because it is in the context God intended it. for our spouse. While a passionate kiss for someone else would be unholy. This is why Paul gave this gave this restriction.
I think the person you are dealing with is going to reject the idea of kissing being adultery and other might support him.
The way I would approach it is this: One, this is not holy behavior for a married man. Two, it is defiantly a step towards adultery. Three even a non Christian would say what they are doing is not wrong for a married person. Approach this like any other sin. Tell him, and then take two or three more if he does not repent. Then if he does not listen, tell it to the church.
So in short is this adultery? I believe you will get a lot of fruitless arguing on both sides if this is the question. Now, is this unholy, sinful activity for a married man? Absolutely!