What guidance is offered regarding Muslim men looking at non-mahram women?
1. muslimahs who are not concerned about hijab?
2. non-Muslim women?

One must differentiate between a harmless look and one which is lustful. The latter is
not permissible regardless of women being Muslim, non-Muslim, mahram or non-mahram.
A harmless look is permissible if the woman does not care about her hijab or does not believe
in it because she is a not a muslimah.
One may look at girls that pass by or simply glimpse one in passing. One may be amazed to suddenly see someone who is half naked but not look again to see if it is really true that she is
so indecently undressed. On a hot day, a few women sunbathe wearing only their underpants. Although the public parks do not arrest or prevent such behaviour, according to Islamic rulings Muslims are not permitted to look at them.
A few youths justify looking with claims that lust is not always the motivation for their looking. They are simply appreciating Allah’s creation of such a very pretty girl. Because their looks encourage and stimulate further interest, they cannot be justified.