Back in Ch. 14, just after Willoughby left Marianne, Elinor was speculating as to why they might not be able to announce an engagment or marry yet:

[Elinor] could easily conceive that marriage might not be immediately in their power; for though Willoughby was independent, there was no reason to believe him rich. His estate had been rated by Sir John at about six or seven hundred a year; but he lived at an expense to which that income could hardly be equal, and he had himself often complained of his poverty.

It seems that the fact Willoughby needed money was common knowledge back at Barton. If he was complaining of his own poverty, it's not shocking to me that Mrs. Jennings would feel no qualms whatsoever about passing on that gossip.