You may borrow a lulav on all days of Sukkot except the first day (or first two days outside of Eretz Yisrael). On the first (two) day(s), you may acquire a lulav and etrog by having a friend “give” them to you as a gift, even if you will later “give” them back to your friend, also as a gift.
Note If you intended to use someone else's lulav on the first day of Sukkot, it is considered as if the lulav is yours, even without doing the normal acquisition.