Has she tried putting them in herself? I find it hurts me if anyone else puts mine in, regardless of their size. Do you know the brand of balls you bought her? If it's from another company you can't link it, but you could perhaps find measurements online? Silly question, but has she tried lube with them? A lot of balls leave that out of the instructions.
Also are you both aware that smaller balls=harder to use?
Position does play a part in determining how easily those balls slip in! I find it easiest to insert them while squatting or lying flat on the bed with my legs in the air. Be sure to get her to relax her muscles as well.
I have the Swoon balls which are pretty girthy (about 4.75" in diameter I think? I could be wrong) and I find it a challenge to insert them unless I get myself in a nice squatting position (kinda like when you're going to pee.. if you know what I mean!) The same thing applies to when you're trying to remove them.
Of course smaller balls would be easier to insert and if your wife hasn't had children / is relatively young, they ought to provide more of a challenge and would be more effective. c: