With respect, it can't. Is that cherry actually a crab apple. It's too late for cherries, so I suspect that the red fruits are crab apples and the 'rogue' apple is from a shoot off the rootstock of the apple. Really it should be removed, and as low down as possible, if you don't want it to take over. It will naturally be more vigorous than the weeping tree. Interesting though.

An apple can't become grafted onto a cherry tree and and vice versa because they are not of the same family. Apples can be grafted onto Pear trees and different citrus trees, i.e. a orange onto a lemon rootstock etc, and it's also possible to graft more that one citrus type onto another citrus rootstock, say a lemon, lime and grapefruit to create a fruit cocktail tree with three different citrus fruits. But the rule of nature is that they must be from the same family.

Are you sure that the apple isn't growing from an apple tree that's growing between the cherry tree, or that it is indeed an apple? From the picture it certainly looks like an apple. It would be interesting to find out what's going on here though.

I think Hortum is correct and the weeping tree is a crab apple and the large apple has grown from a branch of the rootstock. Our crab apples on our weeping tree are just turning bright red and, as Hortum points out, cherries finished fruiting a while back.

I agree, and there does seem to be a lot of the crab apple wood growing. This will be coming off the root stock of the apple tree and needs to be removed right down to below the graft leaving just the grafted part of the tree to grow.

Actually though unless you particularly want the apples the crabs are prettier!

Thank you all very much for the info. Didn't realise it was a crab apple tree & have passed the explanation of how the large apple got there onto Mum.