Writing YHWH as a stand-in for the full Tetragrammaton (יהוה) is generally permitted, especially in non-sacred texts like articles, books, or notes.

There are several ways they can get around it and still be kosher. Specifically, they don't classify English as a language and, therefore, the Tetragrammaton hasn't really been written down; which is just pilpul and business as usual. Also, their commandment is to not deface the name of God or write it spuriously or in a way that it can be defaced or blasphemed. https://www.jewishvirtuallibrary.org/the-name-of-god

Also, religious people are not really known for following their book in total.

Do you happen to know this guy's name? We can confirm.