The good news is that none of my other customers have been getting any spam email trying to sell them avon information products, so fortunately it was an isolated incidence (however still annoying for the person involved).
That means that my website hasn't been compromised and everything is still safe.
The only thing I can think of is that the customer involved gave her name and email address to the other site by mistake.
So it looks like it's back to business for me and I had better start thinking of some new tips for you all!
I'll write more soon.