I'm going to attempt to answer this (although keep in mind that I'm new).
TL;DR: Muslim women, like Muslim men, are required to play their part in proactively preventing sin.
Different societies have differing standards of dress for women. For example, in some African societies, women bare their breasts ordinarily. However, doing so in, say, Australia would get you arrested.
Among Himba women of northern Namibia and Hamar women of southern Ethiopia, besides other traditional groups in Africa, the social norm is for women to be bare-breasted. -- Wikipedia
All societies draw the line somewhere as to "this is appropriate" and "this is inappropriate" with regards to women's dress. The Qur'an (24:31 and 33:59) draws this line for Muslim women.
As Muslim women, we're required to dress and behave modestly; i.e., we draw the line cautiously. We are required to play our part in not drawing inappropriate attention; in not encouraging fornication, adultery, masturbation, etc.; and in guarding our chastity and honor.
Male virility is considered natural and is valued in Islam. When used appropriately (i.e., sex between husband and wife), it makes a significant contribution to the quality of the marriage and to the family unit. However, it can also be misused. As such, men are also required to exercise control and play their part in preventing sex-related sin.
Tell the believing men to lower their gaze and to be mindful of their chastity: this will be most conducive to their purity – [and,] verily, God is aware of all that they do. -- Qur'an 24:30
It sounds like the woman you met was behaving appropriately (and she was even doing a good deed), so I'm led to believe that any sins are not hers.
...no bearer of burdens will bear the burden of another -- Qur'an 53:38
(It's a separate and lengthy question as to what aspects of hijab are obligatory; see e.g.: Why scholars disagree on whether it's obligatory for women to veil their faces or not in Islam?)