This is not true. Rhett did not sell any BTCP, the price dropped because he was removed form the github, and because of this a lot of people sold mistakenly thinking it was a bad sign for the project, when it was actually a good thing. Rhett didn't contribute on github anyways, and he was a bottleneck to the project because of bad communication.

Regarding the fork, obviously the price will drop after the snapshot, the Primecoin is then useless too anyone who is just in it for the Bitcoin Prime. Trying to dump right at the snapshot for a good price is going to be nearly impossibe, everyone is selling and no one buying. If you want to sell XPM and not stay in for the Bitcoin Prime, a better idea is to set some realistic targets for Primecoin and stage out on the way up to the snapshot.
If someone doesn't know these basic ideas behind a fork, or is buying XPM blindly because the price jumped up, they deserve to get rekt. Never blindly invest in anything, especially crypto.