His debt to the girl's father is financial, although he had promised to marry her, he should know a financial debt can not be repaid by emotions. If he tries, he will hurt himself, and both of the girls. Living in modern society, he should have enough courage to break his promise, which is unfair to him and the girl to start with, and repay his financial debt with monetary means.
I do not have answer for your question about them being "just friends", I guess it depends on both of them.