It's possible they:
* Learned it in school: History classes often cover the women's suffrage movement, and "suffragette" is a key term.
* Encountered it in other media: Movies, books, or documentaries about women's rights might use the word.
* Have a general understanding of the concept: Even if they don't use the specific term, they might understand the concept of women fighting for the right to vote.
It's great that your friend knows the meaning of "suffragette," even if they don't use it regularly!