Like Shiken said, I don't understand why it's so hard to believe. They were super close friends. Simon/Shimon seemed to really care about Erza. Remember when he was blown away? "Erza... I..." BAM. What was he gonna say? I LOVE YOU?! PROBABLY. lolol. So of course he's going to blindly believe Erza. And "would the Erza you knew really do something like that?" would be enough to change my heart at that moment.
lolololol@nekonatsu. LOLOL. "This is pretty cool..!"