Is it permissible for a girl to marry a young man whose mother is Christian, knowing that she encourages him to adhere to his religion?

The righteous man is not harmed by his mother being a disbeliever, as Allah Almighty says: "And no bearer of burdens will bear the burden of another." It is advisable to inform your family of the man's desire to propose so they can inquire about him. If they are pleased with his religious commitment and character, then accept him after performing Istikhara.