She is a capable and reliable assistant.

Literal

She [topic-は] capable-and dependable assistant is.

有能で頼りになる — 有能 (ゆうのう, 'capable') connects with the te-form で, and 頼りになる (たよりになる, 'to be reliable/dependable') further modifies 助手 (じょしゅ, 'assistant'). Two qualities joined smoothly.