Is assisting a liquor merchant by writing down his name on his personal phone considered a sin, and is the helper like the doer?

It is not permissible to help your brother record the number of a wine seller to contact him for buying wine. This is because aiding in anything forbidden, in any way, is not permissible, due to the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty."