She’s a legal adult using her own money to buy a product…her mother should have no involvement in this process. 

At what point does a parent stop telling their kids what to do? It really depends on the family that you’re in. Some parents never tell their kids what to do, even when their kids are really little. They frame things in terms of choice, asking their toddlers, «Do you want to brush your teeth first, or read a bedtime story first?» This can be a risky strategy, especially when you’re giving your kid choices that they shouldn’t have. You can’t ask, «Can you eat your broccoli?» without «No» being a valid answer. 

At the other end of the spectrum, some parents never stop telling their kids what to do, even well into adulthood. They will criticize and instruct their adult children on what they should be doing with their careers, relationships, and money, even though their child is 35 and should know what to do in those areas by now.

In my experience, my parents stopped telling me exactly what I could and could not do after I graduated from high school. There were still some things that they forbade (driving in Downtown Dallas was one of them), but it wasn’t like I had to ask them permission every time I wanted to hang out with one of my friends. Even before I turned 18, I could get fast food without asking my mom first.