What should I do about my knowledge that a young man is about to propose to an unsuitable girl based on a woman's lie, and am I sinful if I do not inform the young man of this?

You must advise the young man and inform him of the girl's situation as long as she has not repented with sincere repentance, and you will be rewarded for that. If she has repented, then it is better to conceal her (faults). You can offer advice to the young man through one of your mahram (unmarriageable kin), and it suffices to describe the girl as not being committed without going into detail.