A stutter doesn’t randomly appear due to stress, but will get worse due to it. My situation isn’t identical but mine went away for all of middle school and came back when I was around 13 and was entering high school. Obviously kids are very vulnerable at that age and I don’t know how severe her’s is but either way it will certainly damage her self esteem which very likely is already pretty low and will lead to anxiety. It might take some coaxing but speech therapy will help and you as parent can get resources from them as well, much better resources than a google search will yield. It won’t make it go away but giving her strategies to combat a stutter is useful and beyond that teaching her to be comfortable with it is the biggest thing. Developmental stutters this late onset are essentially unheard of so it’s very possible it will go away on it’s own but in any case get her into some speech therapy if she’s willing and make her feel comfortable with it in any way possible