This is very difficult, there are many factors to consider. First of all, you have to consider your age and your ability. If you are still a student, if the gift is too expensive, his parents will think you are extravagant and don't know how to live. If the gift is too shabby, his mother will not care. In short, it is quite difficult. So you must choose carefully. You need to consider it very carefully, so you need to spend some time. I will give you some suggestions based on my experience.
1. If you are still a student, try not to be too extravagant and pay attention to the heart. Ask your boyfriend if his mother usually has the habit of wearing lipstick. If so, buy her a better lipstick and give it to her. It should be a good choice.
2. If you are an office worker with a fixed income, it is recommended that you buy a slightly more expensive gift for his mother, buy a silver ornament, a bracelet, or a necklace, but necklaces are generally available to people of this age, so you can work with your boyfriend to choose a bracelet that she may like based on his understanding of his mother. I believe she will be very happy to wear it. Now many websites also have customized bracelets like agate, which can be engraved with words, which is more thoughtful.
3. Or you can send a box of nutrient solution, not the kind that is good for the body, but the kind that is specially for beauty. Many celebrities drink this now, and it is said that the effect is really good. I have a friend who also drinks it. You can buy a box for her. I believe she will be very grateful to you after drinking it and seeing the effect.