Although i have no alternate explanation to offer, I don't believe it was Fredo who alerted whoever about the impending Roth hit.

1) I take at face value Fredo's words to Johnny Ola in the late night phone call ("You guys got me in deep enough already...you lied to me...don't call me anymore")

2) I take at face value Fredo's words to Michael in the banana daquiri scene ("Why couldn't we spend time like this before?"), and the look of disgust on his face which I interpret as "I'm really pissed off at myself for ever getting involved with Roth"

3) I take at face value Fredo's words to Michael when he said "I didn't know it was gonna be a hit"

As Michael said to Tom, Fred o is "stupid and weak". I believe Fredo was duped by Roth & Ola, and when he realized the error of his ways, sought to pull himself out of the mess he was in by disassociating himself with them.

Finally, the most compelling argument I can offer is that it was in Fredo's best interest to have Roth and Ola dead, so that Michael could never learn of his betrayal.