i think it happens when you beat the elite four
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the sick boy in Pokemon diamond is in canalave city in the house on the very bottom left corner .x
PKRS makes a Pokemon sick and lets it battle better and get more exp points then normalo so it is great for leveling up. You get it by battling a Pokemon that has it then go to nurse joy in the Pokemon center.
in the I.N.N in the canalave city (you must have member card and the boy in the canalave city sick and you awaking the boy with lunar wing)
In Canalave city up ahead the Pokemon center is a harbor inn. There should be a house in there and in it will be the sailor's son. Talk a little time with him them go to sailor that took you to iron island, and he will be able to take you to new moon island and there you will find the legendary Pokemon cressalia as well as an item to cure the sick sailors son.
You must be talking about the episode where ash and tracey get sick from a poisonous Pokemon and that episode is called the stun spore detour