That stringer needs to be fixed, no doubt about it. It is the structure to the boat; like your bones are to your body. In addition to that, any breach in the fiberglass on the stringer will allow water in, which will rot out the rest of the stringer and possibly the other stringers and transom (depending on how they are all tied in together). If you want to see the end result of what a busted stringer will do, look for my thread in this forum about my full restore. I found a busted stringer behind a half-azzed hull breach repair and along with it I found every other stringer rotted to mush, a failing transom, and a rotted deck (granted, my boat is nearly 30 years old so I'm sure it's wasn't ALL due to the stringer, but still...)