Board Thread:Questions and Answers/@comment-27003142-20160217210943/@comment-27827802-20160219140239

Either way, whether Flowey moved Frisk to a different "dimension", or somewhere else in the Underground, Frisk most likely didn't get past the barrier, as he wasn't able to collect Asgore's soul to pass through the barrier, which requires both a monster and a human soul as stated by Alphys.