In the Name of Allah, the Inspirer of Truth.
Assalamu Alaikum Warahmatullahi Wabarakatuh

It is not necessaary for Muslim women to wear black attire (outside of the home) per se, other colours are permissible as long as they are not attention-seeking colours.

These types of questions are very common. Some people tend to question in this manner to passively create doubts in us, about our faith. You might also like to pose a question as politely as possible to your friend, that do some Christian women in the West not feel cold, dressing the way they do? or something similar to question the way they dress. I think it is a perfectly reasonable question. After all, you gave her question a reasonable response and it might get her to think and question their society.

And Allah knows Best
Wa Alaykumussalaam Wa Rahmatullahi Wa Barakatuh

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