How did elizabeth learn about darcy's role in convincing charles bingley to not become romantically involved with jane bennet?

Answer;

-Colonel Fitzwilliam told Elizabeth that Mr. Darcy had saved Charles Bingley from an imprudent marriage.

Explanation;

-Elizabeth encounters Darcy and his cousin frequently in her walks through the countryside. During one conversation, Colonel Fitzwilliam mentions that Darcy claims to have recently saved a friend from an imprudent marriage.

-Elizabeth conjectures that the "friend" was Bingley and the "imprudent marriage" a marriage to Jane. She views Darcy as the agent of her sister's unhappiness.