Derealisation, depersonalisation

So a friend of mine told me that he has been smoking weed and had moments of derealisation and depersonalisation.

It lasted also couple days after and for now is mabagable.

I would like to help him but I don't know how.

I told him just to try to stay present as much as he can and try puzzle and coloring to help him focus so his mind doesn't wonder,also when he is doing smthg like brushing his teeth to try to be present when he is doing that.

That is smthg I generaly heard can help person to focus and stay present,and I also told him that I'm here for him if he needs anything.

Did you ever had this kind of problems, I would like to help him but I don't know how.

Do you think this might have been product of weed usage?

If you have this type of disorder it would be useful to give me some advce,how can I help him.

He has also been having problems with egsistential thouhts,and his point of view is quiet nihilistic. For example: " why would I even want to go to college when I will die anyway and when I die it will not matter."

Also he is agnostic,so trying to say to him anything about God and after life I don't think would help.