How does Bunky handle that situation?

This depends on what chapter you are referring to. Eldon and the old man met many years before, in a bar called Charlie’s. The old man, then called Bunky, offered Eldon a job and a place to stay while Eldon sobered up. Angie, Frank’s mom, was living with Bunky when Eldon came to work at Bunky’s farm. Eldon effectively “steals” Angie from Bunky, as Eldon and Angie fall in love. Eldon brought Frank to Bunky when Frank was just a week old, and Bunky agreed to care for Frank; Bunky does not like Eldon at this point, but agrees to take Frank in on account of Frank being Angie’s son.