It'll be sooty mould, which usually appears because there's an infestation of aphids. The aphids produce honeydew, and sooty mould grows on the honeydew if there's enough of it. The other cause can be scale insect infestation, so your neighbour should check the plant for both insects, treat, and sooty mould will go away, although it can be washed off with mild or horticultural soap and water.

Thank You Bamboo. I will have to treat it for him as he is in his 90's and I expect he won't bother with it :o))