。
She'll probably become able to play the piano well.
Literal
She [topic-は] piano [object-を] skillfully can-play [come to do-ようになる] [probably-でしょう].
~ようになる is the standard 'become able to / come to do' construction — a change-of-state from non-doing or non-ability to doing or ability. Here it pairs with the potential form ひける ('can play'), giving 'come to be able to play,' a classic learning-progression frame. Adding でしょう ('probably, I bet') softens the prediction — the speaker isn't certain but expects this outcome. Together the sentence reads as a teacher's or parent's optimistic forecast.