Typically 1 x 3 (3/4" x 2 1/2") is the sizing for battons. 1/2" x 1/2" would be way to small Why not have him rip the Hardie Trim boards to size, same as you would need to do if it were wood?

What size do the batten strips look like in the picture? It didnt look like the proportions were 1x3 but its hard to tell. I asked if he could rip the trim boards, but I think he said they could break easily.

Thanks for the suggestion...that seems like the best way to do it. I wasn't sure if there was a typical solution to this