Actually, Ryoga drowned himself (in this fic) in an uncursed
Spring. The guide saw this and notified the authorities who were
familiar with Jusenkyo and retrieved Ryoga's body from the
now-cursed Spring. They returned it to Japan. (note: The
retrieval probably took place after P-Chan was knocked into the
water) He actually posed as his own identical cousin and ID'd
the body himself, probably to spare his family the pain of his
suicide.
P-Chan was knocked (by Ranma) into the Spring of Drowned
Ryoga.
The difference in the curses is the temperature of the water
used to activate them.
Ranma's curse turns Ranma (the true body) into a female when he
is splashed with cold water.
P-Chan's curse turns P-Chan (the true body) into Ryoga (the
cursed form) when splashed with hot water.
So essentially the curse is reversed in P-Chan's case.