Let's say a tester is to do some programming to create automated tests ... is Lua really easy to learn for someone who is not a developer?
It depends on the particular non-developer in question. Some people will utterly block on any programming language at all. Some will easily grok many languages and basic programming concepts. There is no silver bullet for putting the power of programming in the hands of someone who is untested on it.
That being said, my personal feeling is that Lua is as good of a place to start as any other programming language.