For my younger son, I don’t think taking Latin 1 over again would have improved his GPA except possibly the freshman year grade. Older son had no issues. My younger son started Arabic from scratch in college. He took a summer course before starting which helped not at all! He got a C the first year, went to Jordan for the summer, got a B the first semester of second year, but then slipped again. Then he spent a whole year in Jordan and got A’s after that. He’s convince that for him to learn a language he needs to be immersed for a good length of time. I had a very similar experience, but I did discover that second languages come a lot easier than the first one.
As for the OP, I think the teachers, both at the high school and the middle school will probably have the best advice. And I’d ask the kid too.
The advantage of going into Spanish 2 is obviously that even if the grades aren’t great, you can get it over with sooner and take courses you prefer.
I’ve studied a bit of French, Italian and Spanish. I agree Italian is the easiest because of the spelling, but otherwise I’d say they were all pretty similar in difficulty. German is noticably more complicated, but I liked it. Chinese I could not deal with because I can’t hear the tones.