When can I ignore related work in my paper?
Did the paper you're asking about contribute to your knowledge of your topic, or to the analysis or contextualisation of your results?

If yes, you should include it, wherever you found it and whatever it was originally. You might have to qualify your use of a non-conventionally or accidentally published source (if Google Scholar scooped it up from a random webpage the author might not know it's out in the wild). But attribution of sources is non-negotiable.

If no... well it might be up to your judgement. mts's comment about calling it a "related discussion" might be worth considering. If the typical practice in your broader field is to mention significant literature by way of contextualising your own work, I would probably leave this one out (due to the aforementioned questionable provenance). That particular document is unlikely to be a widely significant piece of literature. If the typical practice in your broader field is to mention as much as possible, you might be inclined to include it. A lot depends on how your field typically does things.

Which makes me think: have you been as thorough as possible in checking the author's body of published work, to make sure the same material can't be found somewhere more reputable? If it can, use that instead.