Limits are determined by a variety of factors. Adding her as an authorized user however, should increase her score, if it is not excluded by the abuse algorithm and assuming it has a solid payment record as you stated. Higher score is a factor that can help her to receive a higher SL.
To increase limits, use the card and PIF. If her starting limit is not high enough, that’s what she’ll have to do. But overall patience, it takes time even if she doesn’t PIF. And don’t forget, you can always ask for a CLI, some believe in doing so every six months.
