Is a brother who does not pray and does not manage his spending well, and has previously borrowed money without his situation improving, considered among the foolish whom we are prohibited from entrusting with money?

You should advise the brother who does not pray and squanders his money. Prayer is the pillar of religion, and abandoning it is a great sin. Money is the sustenance of life, and Allah has forbidden its extravagance and waste. You should utilize his need to call him to righteousness.