My friend is canadian and lived his whole life here and went to HS in Hamilton but went overseas for university for his first year in commerce. I was wondering if he can apply to mcmaster and get accepted to 2nd year commerce and how this can be done.

Hey Reda, yes your friend can apply to McMaster. The first step would be applying through the OUAC:
http://www.ouac.on.ca/ouac-105/

Your friend can apply to mac, but i am not sure what year they will put him in, he has to speak with the admissions here at mac. I heard it's a bit competitive to get into 2nd year commerce at mac, so i am assuming that it will be the same for a transfer student (that he has to get a good gpa in first year).

Tell your friend to apply first through the OUAC, then he should get in contact with mcmaster. Here's a contact list:
http://registrar.mcmaster.ca /contacts.html