I'm not sure if it's a typo, but I'm not clear on the kicking a ball part - he kicked a ball so the other kid was hit by ball and fell? Were they playing together? A sport? Why did the mom wait 2 weeks? Or was she upset about the book incident and brought up something else in the past? If so, that seems kind of odd at least to me.
I'd let that go. Anyone who waits two weeks to bring something up, and get upset about a child not lending another a book ... seems a little off. My kids might not lend a certain child a book/toy/something ... that's up to them, and that's their choice. I don't always feel like lending certain people (even certain friends) books. Depends on how they care for things, or if they are 'special' to me.
I have a neighbor who used to come to me with her looking out for her kid, and always complained that mine were doing stuff to hers. They weren't. At first though, I questioned if they were each and every time. No one else had every mentioned anything, and I had never picked up on anything. Whereas this mom seemed a little 'off'.
It was the mom. Turned out she did this to everyone else and their kids.
So trust your gut, and just remind him to take care, and just observe him if it happens again. See how he handles himself.
All kids have off days, and not all express a lot of empathy at that age (and during teen years). They will look out for friends in general, but can be hard on each other too. Friendships change too - and remember they are not opening up to you as much as they once did.
I once had a mom call me to complain my son wasn't kind to her son, who she felt was her son's friend, and it turns out her son had dumped my son as a friend the week before. Parents sometimes get it wrong.