What should I get a twenty-something married woman for her b-day?

I don't know anything about this woman other than the fact that she's married. I have no idea what she or he would like. You are the one who is friends with her. Think about her interests and her likes and get her something she would enjoy. Everyone's different. Not all 20 year old married women like the same things.

So her only defining characteristics are that she's married, a woman and in her 20s? She might like exactly the same thing as a single woman in her 40s or a man in his 70s.

If it's her birthday then the gift is for her, not her and her husband.

Well, if it's her birthday, look for something for herself only. Couples gifts should only be given on occasions celebrating the couple. That should be a good start.

You could get her a kitchen utensil which she doesn't have. Useful for them both to cook & eat.