What is the ruling regarding the fiancé of one's sister who flirts with women while he is praying, and has been advised many times?

The questioner should advise his friend that flirting with women is dangerous to his religion and character, citing the hadith: "I have not left behind me a trial more harmful to men than women," and the words of scholars regarding the prohibition of speaking to a young foreign woman without need, because it is a source of temptation. He must not underestimate these sins, for evil deeds invalidate good deeds, as stated in the verse: "O you who have believed, obey Allah and obey the Messenger and do not invalidate your deeds." If the friend repents, then his marriage to the sister is permissible; otherwise, he should not be recommended.