Must a woman accept a proposal from a man who already has a wife?

No — a woman is free to accept or refuse any proposal, including from a married man. A woman's consent is the foundation of marriage; the Prophet ﷺ forbade a widow or divorcee to be married off without being consulted, or a virgin without her permission, and he annulled the marriage of Khansa' binti Khidam, whom her father had forced. Refusing to become a second wife is not rejecting the Shariah; polygyny is permissible, not obligatory. Weigh it up through istikharah, family advice, and an honest assessment of the man's religion, character and means. See prospective second wife.