If my friend is kafir and touch for exemple the door with her wet hand how I can purify the door?
If I take a wet sponge and clean the door, the wet sponge become najis as well?
How to do?

For najasat (excluding urine) you need to remove the essential impurity then if using small amount of water, like from a water bottle for example, you pour one time and it is purified.

Yes in your case if you used a wet sponge to clean the impurity it would make the sponge najas.