。
It's preferable for her to do it alone.
Literal
She [topic-は] alone do [nominalizer-の] [subject-が] desirable.
の after a verb here is the nominalizer — it turns the verb phrase ひとりでやる ('do it alone') into a noun-like clause that can take a particle like が. Japanese has two main nominalizers: の (more colloquial, leans toward describing a specific event) and こと (more abstract, often for general truths). 望ましい ('desirable, preferable') is an i-adjective derived from the verb 望む ('to wish'), one of a small set of adjectives describing how something ought to be (例: 好ましい 'agreeable,' 喜ばしい 'gratifying').