he’s using google chrome

That answers how it happens. My guess is that he googled things relating to submarines or his browsing behavior regarding that game caused him to be grouped as potentially interested in submarines. If you are using chrome everything you do will be collected and used for marketing. No amount of extensions will save that since the browser itself is spying. If you want to minimize this happening you have to switch browser (brave is the closest to chrome) and stop using google for search.

“Looks like you have a suicide scheduled at 10. Here are some tips I found on the internet!”

So you use Instagramm but are concerned that Threads collects too much data? Next you are going to tell me that you don’t use Facebook because you don’t trust the company behind it?