It's not clear that Mauritius has such a strong case here. The history is complicated, but "On 5 November 1965, the Governor of Mauritius informed the Colonial Office as follows: Council of Ministers today confirmed agreement to the detachment of Chagos Archipelago"[0].
Of course now the current Mauritian government is claiming that it didn't really mean to agree to this detachment, and it's certainly possible that the Mauritian government in 1965 felt that its independence process (completed in 1968) would go more smoothly if they agreed to give Britain territory for a military base, but that doesn't mean that 50 years later they can try to reverse history (and claim for themselves a nice rental fee from the US for the island).
