If it were me, if my best friend started to exercise, I might give her a sports bra, and this kind of thing needs to be washed when sweating, and it can be bought at a price of about 200. If she has something that can be changed, I believe her heart will be beautiful; If the budget is limited, consider sending a yoga mat or something.
If you are good friends, you'd better not go to Decathlon (a sporting goods supermarket with convenient shopping) to pick a decent sporting goods. If the budget is limited to 100 or even less, you can buy a sports kettle or an arm bag that you can wear on your arm when running. If you can accept something more expensive, you can look at sports protection tools at home or small devices that can simply roll your stomach ~ Of course, if you have a good relationship, you can also consider protein powder and the like, but it depends on your friends' preferences. I feel that some boys like to drink this very much.
Hope to adopt ~ I wish you a good mood in advance and a happy birthday to your friend ~