Why a Buddhist prays at Hindu temple which is situated in another country and prays for the betterment of his own country?
Maybe it's a politic and skilful thing to do -- analogous to the Prime Minister of New Zealand wearing a headscarf after the Christchurch mosque attack.

New Zealand PM Jacinda Ardern, wearing hijab, meets Christchurch terror attack victims' families

Perhaps it helps to represent that (the population of) Sri Lanka is Buddhist and Hindu too, and that populations (within countries and between) can co-exist peacefully.

That's just a guess though, I haven't read any publicity about his motive.