I will comment, there's a difference between having a pad on stage for occasional reference that isn't distracting, and total reliance on singing from a pad where it kills any stage presence. I don't have a problem with the former. There's also a spectrum from "not knowing the lyrics" to "occasionally flubbing a lyric." I sing "The Middle" in Mugshot Monday and the verses are so similar, it's easy to flub them - is this one "DO your best" or "TRY your best?" That kind of thing, live, doesn't usually matter much.
It's unclear to me if the OP's singer's problem is difficulty learning lyrics or just that they don't put in an effort to learn lyrics.
A couple of tricks I'd suggest for certain issues - I don't know if vocal teachers or anyone have official names for them, but I'll call them "leading words" and "subtext narrative."
By "leading words," I mean that in some songs what I needed to do was just to drill the first word or phrase of each verse. I could get muddled, but if I hit that first word the rest of the line would kick in. For instance, doing "Wagon Wheel" (I know, I know...) I used to get tangled over what the verb was leading each verse, and then stumble over getting through it. So I drilled into my head, "HEADING... RUNNING... WALKING..." and the rest fell into place.
"Subtext narrative" is my term for creating a logic to songs where the lyrics don't have an obvious surface logic. There are some artists - Yes and RHCP both come to mind - where the lyrics are often abstract word-paintings and lines seem to come in a jumble one after another with no apparent connection. For songs like that, I try to come up with a story line - the narrative part - that underlies the words (the subtext). For instance, for RHCP's "Can't Stop" I make the first verse about an indigenous kid who's relocated to a big city and gotten mixed up in an underground criminal scene.