Hello and welcome to healthcare forum.
In major surgery like mitral valve replacment, during retraction of sternum
bone it is sometime possible that one or more ribs get broken. The broken ends of the rib may cause trauma to an artery
nearby and leads to bleeding.
So, what happened to your father is known complication and what operating surgeon did is also right.
As the source of bleeding is under control you should not worry about it.
Best wishes for speedy recovery of your father.