I agree, this is a big red flag, and as you said the visa application was rejected which implies he went to an interview at the consulate and was rejected for the visa by the consulate officer. If that is the case, he knows why the visa application was denied, and it is not uncommon for K1 applications to be denied from MENA countries. Since this petition was from another USC, there is no way you can get a copy of that information, so you only have his side of the story. As this is a social media site devoted to legal immigration, we have no way to determine your relationship with him, but I would be very careful.
Good Luck!