My thoughts from a strategic perspective:
1) If she wants to take multivariable calc in high school, she should think about taking it next year. It's very easy to forget math if you don't use it, and she probably won't want to spend a summer reviewing limits and the chain rule, on top of all the other summer homework she's likely to have. Also, multivariable is one of those subjects (for me at least) that have a way of making it all come together. By this I mean that I started to see how a lot of ideas in mathematics fit together after being exposed to multivariable calc. Biochemistry had the same effect in the biological sciences.
2) My understanding is that a year or more of statistics is typically required for a psychology degree. So if she wants to major in this subject, IMO, she'd do better to take first semester stats in her senior year. If she wants to enroll in a second-semester stats course in college, again, she'd make things easier on herself if the information from the first semester course was fresh in her mind.
Basic stats is not the hardest class on the planet --- especially after two years of calculus.
If she's going to be devoting a lot of time to college applications, having an easier math course in her schedule might help reduce stress.
