Hey Trixie,
That’s a really nice thought. I think it’s wonderful that you’re thinking of your friend and want to be supportive and show her that you care.
I don’t know if there’s a hard and fast rule about sending remembrance cards and so I think this is both a personal and cultural preference.
In North America, I don’t believe it’s a custom to send remembrance cards so I wouldn’t do it myself. In some European countries, I understand that the first year marks the end of mourning for a close loved one so I don’t know where a remembrance card would fall into that tradition.
The best advice I could give you is to just ask her about how she’s planning to to commemorate the anniversary. Some people visit the grave of their loved ones on special occasions (anniversaries, birthdays, holidays, etc) so perhaps you could offer to join her. For some people, past the funeral, any mourning is done privately or with only immediate close family who knew the deceased.
If you don’t know her that well and as you say, you’re “sort of social friends”, this would be a good opportunity to talk and get to know her personal preferences.
“Do unto others as you would have them do to you.” is a beautiful rule, but I would also invite you to consider “Do unto others as they would do unto themselves.”