@Павел-Ермаков"The least she can do is…" is an idiom meaning "She must, at least…". In the past tense, "She had to, at least… " The verbs which follow are infinitives, just as they would be in Russian, that's why 'use' is not conjugated.
@dev_temp 'To' is only optional in this sentence. It has to do with 'could', being a modal verb. Whether it's officially correct, I don't know, but cf. the song "All I could do was cry". The construction in the link is something different. e.g. "She was to use…" means "She eventually used".
@maksim всё что могла она сделать, это возместить ущерб по страховке
Well that's a bad match for the English sentence. If I were to translate that back into English, I would say this has the opposite meaning: "The most" she could do was…" or "All she could do was…"