。
She looks much younger than her husband.
Literal
She [topic-は] husband [compared-to-に比べて] very young looks.
Three pieces in play: ~に比べて ('compared to') sets up the comparison, とても ('very') intensifies, and ~く見える ('look [adjective]') is the standard frame for visual judgment. ご主人 is the polite term for someone else's husband — note the honorific ご prefix: you'd never call your own husband ご主人, just 主人 or 夫 in talking about him to others. This in-group vs. out-group register shift is one of the most distinctive features of Japanese kinship vocabulary. 若く is the adverbial form of 若い ('young'), pairing with 見える ('appear, look') to mean 'look young.'