I grew up with a guy who went on a trip to either Thailand or Indonesia (I forget which but I believe this happens in both places), a sports team trip, they all went out for dinner/drinks at bars, he wasn't feeling so good at one point and went back to his hotel room. They found him dead in his room the next day. It was because bars were essentially using 'homebrew' spirits in their drinks. He was not the first, he is not the last. I'm not sure how that relates to the causes of death of these two, I'm not sure of the official cause of death of the guy I knew. I don't know too much about the Seychelles but I know that in Thailand/Indonesia the advice is to think about how that drink is going to be made - beer where you see them pop the top is best, even if you see them pour vodka out of a bottle you recognise - well, it could have been refilled with homebrew. I'll see if I can find any articles, in the last couple of years there was a little spate of incidents.
Here's one article:
https://www.crikey.com.au/2013/01/10/the-dangers-of-cocktail-hour-in-kuta-why-does-arak-kill/
Again, I'm not sure of the drinking culture of the Seychelles but this was the first thing to come to mind since it's 2 people at once.