To light a Friend's Wishing Torch, open the Friends Menu.

If the Torch button is highlighted, you can light your Friend's Wishing Torch directly from the Friends Menu by selecting the highlighted button.

If the Torch button is not highlighted, selecting it will open the Map so you can visit one of your Friend’s Islands and manually light any unlit Torch by selecting it.

If the Torch button is darkened, your Friend has no unlit Wishing Torches available to light.