I think most of us have felt some of his pain at least in the beginning!

This is a "brutal survival game" - unlike a lot of other survival games that say they're hard, this one truly takes a lot of patience, planning & preparation to progress with the least amount of this kind of frustration. It does not cheat, there is no mechanic to mob spawn your tombstone... it is just unforgiving if you try and rush around without a solid plan, or backups to your solid plan. Husband would be better off slowing his roll and figuring out a new approach so he doesn't have this experience. Most people only die when they make a mistake - I know in my case every one of my deaths have been entirely my fault one way or another.

This is a recent thread here where a LOT of great strategy and tactics were discussed, it might be useful to take a read through! https://steamcommunity.com/app/892970/discussions/0/3035976838032130518/

Or, if he's really stuck, there are some great guides written by other players right here - https://steamcommunity.com/app/892970/guides/

Hope some of this helps his blood pressure & your peace & quiet

Edit: If the last boss he fought was Bonemass - the swamps tend to be quite tightly packed with mobs, some pretty fast moving ones too, and generally smaller areas than the others (at least in the maps I've played on!). So I can see why he would feel mobs were all over his tombstone lol