。
She's a perfect match for her son.
Literal
She [topic-は] son [to-に] suiting [genitive-の] marriage-partner is.
似合いの結婚相手 — 似合い is the noun form of 似合う ('to suit, match'). The structure ~に似合いの[noun] gives 'a [noun] befitting / suiting / well-matched to X.' 結婚相手 ('marriage partner') is built from 結婚 ('marriage') + 相手 ('partner') — same productive 相手 family seen in 話し相手 ('conversation partner'). The whole sentence is a parent or relative endorsing a son's romantic match. (Or — perhaps with an ironic twist — sizing up a match.)