I had a similar situation when a customer said i had scratched her patio door, which i 100% knew i didn't do, she threatened me with solicitors and allsorts, the advice i got given at the time from peeps off here was that she didn't have a leg to stand on, if your customer takes legal action they will be asked what proof do you have ie; did they see you do it, even then it's your word against theres.
Just forget about it and move on don't get into arguments with them, i take it they don't want you to clean their windows anymore?