Honestly, this kind of thing happens more often than you know. Mario Lopez, while in A Chorus Line, didn't have an earpiece but did read from his script in the back of the house, and had notes written inside hats and posted on walls just offstage.
Sometimes even that wasn't enough, and he'd just paraphrase.
Zach's "Listen, now listen. Larry has exactly the style I'm looking for, very thirties. Everyone keep your eye on him. Let's do it again. A five six seven eight"
became, with Mario: "Ok, everybody check out Larry. Five six seven eight".
My first choice would have been to have someone else in the role. My second? Have him wear an earpiece.