Another example of a device that would be a problem like this is my watch. Jewelry that would have similar problems is also common. Things which would be a problem if put close enough are everywhere, but this is always going to be an issue with something like this that is designed to be triggered easily and extremely reliably, and the decision has previously been taken to educate the tiny number of users of them about it rather than go through the unimaginable logistical effort of changing them now (obviously it was much easier when they were new and rare).
Edit: removed paragraph about making a warning device that I don't think is worth including here.
If you are wearing a suit, a smartphone makes the trouser pockets bulge a lot, so it's common to keep it in your shirt pocket or jacket inside pocket - Both of which is next to your heart.
1: https://www.fda.gov/radiation-emitting-products/cell-phones/... 2: https://www.heart.org/en/health-topics/arrhythmia/prevention...
Same with smoking, drinking, taking drugs, etc.
He doesn't have a pacemaker, but it may not be as unreasonable of a thing as you might assume.
ex: https://oldnavy.gap.com/browse/product.do?pid=3397220020504&...
It's the most convenient place for a phone on your person.
Easier than pants pockets if you're sitting down. No fumbling with inside jacket pockets. And you can use your "normal" jacket pockets to keep your hands warm without worrying about your phone falling out.