The others are right. Motivation is key. As is the time commitment.
As a student in public high school (put there for math alone after homeschooling for 5 years), I took Geometry and Algebra 2 together. Loved it. Wonderful year. I was VERY motivated and ate math like it was icecream. My peers were not doing the same. I fell asleep on my books every night. I had no other life–but didn’t want one!
Geometry requires some mastery of the concepts in Algebra 1. So I would not normally recommend those two together.
That said, I did teach a homeschool coop for a small group of very motivated (close to graduation) students where I taught them Geometry and Algebra 1 concurrently to catch them up. The time commitment was huge. The effort was tremendous. They worked very hard. I was preparing them to go straight into College Algebra, which they did with success. However, I was able to do this, because I was right there, stopping in the Geometry when I needed to and relating it back to the Algebra. I was on board and held their hands and helped them over any spots. I was also very hard on them, so that they could succeed in the college class they were desiring. It is not something I would recommend in more normal circumstances.
I agree that another valid option could be the above, where you just do the math sequentially through the summers, too. It might be less stressful and less effort/time commitment at one time. Depends on your student and where her focus is…and what the rest of her load is.
Hope that helps,
Joanna