What Causes Appearance Of Large Bruise On Stomach And Back Area?

My brother in law started with a bruise on his side, the size of a mans shoe. No one knew where it came from, he was in the hospital at the time. They told him don t worry it will go away. That was three days ago. Now it has spread across his belly, down his back into his butt crack and down his leg. It is the boggest blackest bruise my sister has ever seen and now she is worried. What is it and should she be worried?