。
She must have completely forgotten about that promise.
Literal
She [topic-は] that promise [about-のこと] [object-を] completely forgot-ended-up [must-be-にちがいない].
~に違いない signals strong inference, the speaker reaching a confident conclusion from circumstances rather than direct knowledge — close to English 'must have' or 'no doubt.' すっかり ('completely, thoroughly') is a 擬態語 that pairs naturally with state-change verbs like 忘れる ('forget'), 治る ('heal'), 変わる ('change'). The auxiliary ~てしまう adds a regretful, unintended completion: 'ended up forgetting,' not just 'forgot.' のこと after 約束 turns the promise into an abstract reference, the kind of phrasing that pairs naturally with verbs of mental engagement (think about, talk about, forget about).