How do you make a door so you can walk through it?

Hi there. In Roblox Studio, click the door. If it's just one part, click the part you want to be trespassed. If they're more than one, click the part(s) you may want to be trespassed. Once you clicked them, take a peek at the object's properties. You may see these two right under "Behavior":

Anchored CanCollide

Make sure you tick "Anchored", but untick "CanCollide" this way, the block won't collide with anything, but won't fall off the ground. You should pass through the door properly.

PD: If the door is a model, you'll need to as well explore it until you see the part you want to be trespassed or ungroup it and select those parts.

Luck to your friend!

Aimarekin.

And, too, if it helped, accept me as answer ;D LOL