Just because a woman is married doesn't mean her social life is over. Married women can still have active social lives and plenty of friends, and these can exist apart from her husband. If you are giving a married female friend a gift, consider her likes and needs. If you are a male, you will want to keep your gift appropriate, and perhaps consider your friend's husband or children in the gift so that it is respectful of her family.
Choose a book that is specific to your married friend's interests. If she likes gardening, cooking or loves art books, find something you think she will like. Giving a classic novel is also a safe bet. Wrap the gift and give it to her on her birthday or at the holidays. If you're a coworker, give her the gift for her anniversary of working at the company so it doesn't seem out of the blue.
Give her a gift certificate to a restaurant you know she likes, or one that she has been wanting to try. Reward her if you know she has had a particularly hard month or recently finished a grueling project. If you know she has not had much time to spend with her husband, hand her an envelope and suggest she and her man take a load off and have a date.
Help out a married friend by giving her the gift of time. Offer to take her kids for an afternoon, help her clean out the garage or help her organize and update her computer. If you know she is overwhelmed by the chores of married life and kids, she will be extremely grateful for your help. Give this gift anytime by calling up your friend and saying you have some free time and would love to help out.