From the Facebook page:

A primary challenge against Donald Trump is a very bad idea. If you come against Trump from the center, you will get destroyed. The only hope you have is coming at him from the right. Then, one of three things happens:

1. You get crushed, discrediting conservatism within the GOP and possibly moving Trump to the left.

2. You lose, but come close enough to damage Trump, causing him to lose the general election.

3. You win, but the party is now so divided that you lose the general election.

All of these outcomes are very bad. This is a no-win scenario.