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She's knitting a sweater for her boyfriend.
Literal
She [topic-は] boyfriend [for-に] sweater [object-を] is-knitting.
編む ('to knit, weave, braid') is a tactile verb covering knitting, braiding hair, and even editing (a story or anthology) — the unifying image is 'putting strands together into a structured whole.' に here marks the recipient or beneficiary — 'for her boyfriend.' Knitting a sweater for a boyfriend is something of a culturally loaded gesture in Japan; there's a folk-superstition called 'the sweater curse' (セーターのジンクス) — that gifting a hand-knit sweater to a boyfriend will lead to a breakup, a notion shared with Western knitting communities.