Her inncocent ball of life will eventually be tainted by swear words anyway. He just has a 9 year head start. I don't think ignoring the word will help... if they explain it and explain why it's so bad, he won't ever used it.
When I was 10 I was reading the blurb of South Park DvDs and saw the episode "Cartman's mum is a dirty slut". I thought it was simply a fun insult for a female. Unfortuntaly during a game of marbles with my sister (as you do when 10), when she beat me or did something amazing, I called her a dirty slut, needless to say I was shocked at the reaction, and yet I still didn't understand what it meant intill I was a teenager.
Basically, the best thing the mum can do now is explain what the word means and explain it's very offensive. Otherwise the bright child will remember the word and use it out of context for horrifying results.