Put the sugar and 250 ml of water into a small saucepan and bring the mixture to the boil, stirring until the sugar has dissolved. Reduce the heat and simmer for 5 minutes to make a syrup. Strain the lemon juice into the syrup, then set it aside for 15 minutes to cool.
Purée the syrup, canned mango and coconut in a food processor, or with a hand-held mixer, to make about 800 ml. Pour the mixture into a freezerproof container, cover and freeze for 2 hours, or until it is just firm. (Use the fast-freeze setting if your freezer has one.)
When the mango mixture is frozen, whisk the egg whites until they form soft peaks. Scrape the mango mixture with a fork to form crystals, then use a whisk to beat in the egg whites, making sure they are well mixed. Return the sorbet to the freezer and freeze for 1 hour 30 minutes.
Remove the sorbet from the freezer and whisk it again. Press it down with a spatula and return it to the freezer for a further 1 hour, or until it is firm.
When the sorbet is almost frozen, prepare the fruit for the salad and put it in a bowl. Add the lime or lemon juice and gently toss it into the fruit, then set the salad aside.
Heat a heavy-based nonstick frying pan and dry-fry the coconut for 30 seconds-1 minute, stirring, until it is lightly browned around the edges.
Serve the fruit salad with two scoops of sorbet per person, topped with the coconut. Any remaining sorbet will keep, frozen, for three months.