In this sentence there isn't a whole lot of difference to me.
"clemency" is usually used in relation to punishment of offenders by the judicial system. "leniency" is often used in the same way, but it can also be used to refer to someone's behaviour or attitudes in a more general sense. Randomly Googled example: "His father's leniency was more than balanced, however, by a mother who was so strict...". In this sentence, "clemency" would seem a bit odd to me because no official judicial process is involved.