I think your mother-in-law is suggesting that the branch may root if you plant it. The only tree I know that will strike roots if planted upside down is the willow. I doubt that your apple will strike roots now though it is worth digging a trench about 6 inches deep, filling it with sharp sand, clean any ragged bits of the base of the branch, set it in the trench and water it. Leave it until spring, if it starts to shoot, you may have got it, don't disturb it, leave it until the following spring. If it then shoots, dig it carefully out of the ground and pot on or dig a hole where it is to be sited. Oh, remove the apples that are on it now and eat them. Growing apples from seed isn't a good idea, they won't come true, and apples from seed can take as long as twenty years to flower and fruit.