If there are rooms available, Disney will let you book a room through them so it doesn't seem as though there's much difference in time, at least, between booking via
DVC and regular Disney. As Bill says, depending on where your friends own (their "home resort") they probably would be able to make the reservation 7 months to the day prior to when you want to go. And, no, it would not cost more if their home resort is Boardwalk than if their home resort was the Polynesian. You should be aware, however, that the people whose home resort
is the Polynesian will have a few months lead time on any other DVC owner so it's likely that many of the rooms will have already been scarfed up by the time your friend can make the reservation. (Not that it's impossible; just that, with anything new, it's going to be highly desirable to a
lot of people just like yourself.

)
So, yes, have your friend check on the DVC points chart to determine how much it would 'cost' you to stay there and try to (assuming their home resort is Boardwalk) make that reservation right at the 7 months prior to your stay date. It won't cost more than if they owned at the Polynesian. And if you are truly wanting to stay there but can't get a reservation via DVC, check the Disney site to see how much that would cost as they may have a few rooms available. Good luck!