I don't believe the BF2 can be accessed via the main mountain area, but I may be wrong on that. There is a BF2 area (Citrine Mountain, however) that can be reached via the cave in the North Aventurine forest, but attempting to dive there caused my Reborn to crash, and I can't say for sure that that's where Lapras could be. Anyone else is welcome to try and see if that's where it is, however, and I'll probably edit in the results of my next attempt