How can one deal with a person whose behavior and Islamic understanding have changed after returning from travel, and who no longer maintains their prayers?

This is the result of committing what the Sharia (definition) has forbidden, namely traveling and residing in lands of disbelief without necessity. The young man must repent to Allah from sins, especially his negligence regarding prayer, because the preponderant opinion among scholars is that abandoning it constitutes disbelief. His family should advise him with gentleness, kindness, and patience, seeking help from people of knowledge and righteousness, and reminding him of stories of disobedient people. It must be noted that it is not permissible for the questioner's cousin's son to advise and guide him if he is not a mahram (definition) to her; rather, someone else should undertake that.