When Colonel Brandon is telling Elinor about Eliza Williams and Willoughby, he says this:

"He had left the girl whose youth and innocence he had seduced, in a situation of the utmost distress, with no creditable home, no help, no friends, ignorant of his address! He had left her, promising to return; he neither returned, nor wrote, nor relieved her."

"the wretched and hopeless situation of this poor girl, and pictures her to herself, with an affection for him as strong, still as strong as her own"

Eliza, like Marianne, is 17 years old. Even now, a year after their affair, her affection for Willoughby is still as strong as Marianne's--and Colonel Brandon knows how strong Marianne's affection for Willoughby is. Willoughby promised to return to Eliza--what else did he promise her, or seem to be promising her, although it was 'every day implied, but never professedly declared'?