Is a woman's helping a man who has fallen to the ground on public transportation, in the absence of another man, considered haram, and would she be sinful for it?

A Muslim woman must not touch a man who is a stranger to her, and he is not permitted to touch her. Whoever refrains from doing so out of fear of embarrassment and sin will be rewarded. However, it is permissible to touch a strange man in cases of extreme necessity, and only to the extent of that necessity.