Don't choose clothes as a birthday gift for your boyfriend's mother, because everyone has different perspectives. If your boyfriend's mother doesn't like the clothes you choose, it will leave a bad impression on her.
Beautiful jewelry is a good choice. No woman doesn't like it, no matter how old she is. My suggestion to you is to customize a piece of jewelry that belongs only to your boyfriend's mother, such as a name necklace. It is made with her name, which is absolutely novel. I believe she will like it.