Is it obligatory to advise a neighbor of high morals who prays at the mosque regarding his daughters going out immodestly, without hijab?

You must advise your neighbor and remind him of his responsibility towards his family and daughters, as Allah Almighty says: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones." And according to the saying of the Prophet, peace and blessings be upon him: "All of you are shepherds and all of you are responsible for your flock." Part of this responsibility is to guide one's family to obedience and to avoid disobedience. A woman's failure to adhere to modest dress is a sin. So, advise your neighbor to fear Allah regarding his daughters and not to let them go out displaying their adornment, otherwise, he is negligent and a betrayer of his flock, as the Prophet, peace and blessings be upon him, said: "Any servant whom Allah entrusts with a flock, and he dies on the day he dies while he was a betrayer of his flock, Allah will forbid Paradise for him."