Reddit reviews Statistical Physics, Third Edition, Part 1: Volume 5 (Course of Theoretical Physics, Volume 5)

By chemist, do you mean undergraduate or postgraduate? What year of study are they in? It'd be difficult to study statistical mechanics from scratch; make sure the following prerequisites are in order:

• Mathematical methods including multivariable calculus, vector calculus, differential equations and introductory (but still rigorous) probability theory. Combinatorial methods can and will help, too.
  
• Classical mechanics, including analytical mechanics. A lot of important results in statistical mechanics correspond directly to what you find in classical mechanics.
  
• Exposure to thermodynamics is essential. As a chemist, your friend will almost definitely have this.
  
• Quantum mechanics, the ideas of which are highly important for quantum statistical mechanics. Of course, if your friend would rather stick to classical statistical mechanics, this doesn't have to be deeply studied. I'd imagine that being a chemist, your friend has seen some quantum mechanics before anyhow.
  
  For an introductory level book, I quite enjoyed Bowley and Sanchez. They go through relevant ideas in probability already and the appendix covers up some of the mathematical prerequisites. Further down the line, Huang is an excellent book: it is significantly more advanced than the previous, but the contents is both broad and detailed (I still refer to it for topics like the 2D Ising model). At the same time, you could also consider Volume 5 of the famous Course of Theoretical Physics by Landau and Lifshitz. The Course is famously hardcore, but it imparts mastery like nothing else.

For an undergraduate approach I recommend Schroeder. However, this book starts with thermal physics which is, well, a bit boring ;). The math is not hard, but developing that 'physics instinct' can sometimes be challenging.

For a more advanced, but very nice and systematic text, I recommend Toda, Kubo, et al.. Another graduate text is Huang.

There are also the books by Feynman and Landau and Lifshitz Pt. 1 (Pt. 2 is quantum field theory, which at this stage you probably will want to avoid).