Elle a perdu une bonne occasion de se taire.
ell lah pair-dew ewn buh noh-kah-zeeYAWD suh TAIR. Click below to hear this pronounced!
She should have kept her mouth shut.
Or, more literally: She wasted a good opportunity to be quiet.
Perdre can mean to lose or to waste. Une occasion is an opportunity, or sometimes a sale (discounts, save money, that sort of thing). Se taire is to be quiet, or more colloquially, to shut up. Taisez-vous is a not-terribly-polite way of telling someone to be quiet.
Seems to me this would be a good opportunity to take my own advice. À demain! See you tomorrow!