There are so many logistical issues that you did not cover in your post, Garth.
For example; is your friend young (in his 20's) or on a second career path (late 30's or older)?
Does your friend plan to dress loosely or more semi-formal in his kilt for the interview?
Is the police force strict conservatives or open minded about kilts worn in public?
Is your friend known as a kilt wearer in the community?

If your friend is competing with many others for the same job, being kilted would get him remembered as bold and inventive (unless he came off defiant to authority). If he is ready to answer any question about his choice to wear a kilt to an interview, I say let him try.

A second call back is usually a good sign for a job prospect.