There are others around here who know far more than me about the Chagos
- but my understanding is that won't be very many cruising Yachts visiting at any given time.
Downside is that the potential numbers of people who could have encountered your brother will be small - upside is that those Yachts will have needed to check in with the authorities. If you can find out that information (personally or via your own officialdom) then can try and track folks down. They may even have supplied contact details (e-mail?). Failing that the Port of Registry would be useful, as well as vessel / owner / crew names....whether that be by then using official resources for contact details or via Mr Google
(for a blog / website). Odds are that most folks will be unaware of the event, and even if they are likely that they have nothing to report - which I guess would nonetheless be useful for you to know.
If the crew was picked up from a previous port could also do the same there (and likely a bigger pool of yachts).
Would also be useful to say what the vessel type / model was - and with a picture of the boat and your Brother (some folks remember boats easier than faces!). if your Brother had a blog or website might also ring a bell for someone.
If you have access to any computer he kept onboard might find out whether he is on any forums
- he may have shared info (concerns?) with others online that is relevant.
But also to bear in mind that if someone goes overboard (and that can happen easily / for the most stupid of reasons
- no matter the experiance of the person involved, indeed "knowing what you are doing" can be part of the cause) then in practice sometimes very little that folks onboard can do, especially if they go overboard without anyone immediately noticing (and also keeping them in sight until recovered). The sea is very big - even someone 100 metres away can dissapear from view, and a boat can travel that distance in a minute or 2. Even half an hour later would be like looking for a needle in a haystack.