I wouldn’t make a _snarky_ comment not would I “snap”, but if 9 months in something hasn’t stuck that should have, I would absolutely communicate that (as well as helping with the problem). As in “here’s how you flerble the foobaz, but FYI I’m a little concerned that this isn’t something you’re comfortable doing by yourself yet”.

Pretending that underperformance is OK doesn’t help people - it’s the kind of thing that leads to them being put on PIP “out of the blue”.