Wickham and Mrs Younge were known to each other before the attempted elopement.

See this from Darcy's letter.

About a year ago she was taken from school, and an establishment formed for her in London; and last summer she went with the lady who presided over it to Ramsgate; and thither also went Mr. Wickham, undoubtedly by design; for there proved to have been a prior acquaintance between him and Mrs. Younge, in whose character we were most unhappily deceived; and by her connivance and aid, he so far recommended himself to Georgiana, whose affectionate heart retained a strong impression of his kindness to her as a child, that she was persuaded to believe herself in love, and to consent to an elopement.