The clue to decrypting this message is written at the top: "the # of letters in your dream guy's first name". Who is Diane's dream guy?

Examine the pictures of Diane with a man ("3 Pictures of Diane"). Who is the man?

Examine the Saguaro Sands Newsletter carefully.

The man in the romantic pictures with Diane is Steve Marshall—Diane's "dream man" is Steve. We know it's Steve in the pictures, because there's a picture of Steve in the Saguaro Sands Newsletter.

So, the clue to decrypting the message is "5" (5 letters in "Steve").

This message was encrypted using a Caesar cipher. That means each letter in the original message was shifted forward by 5 in the alphabet.

Decrypted, the message reads:

I saw you two at the greenhouse on Monday. I know what you’re doing. Come clean by the fourth or I’ll tell Rick myself. He’s my friend too, and I won’t lie to his face.

-Evelyn