Do you agree with my friend about crossing the line?

I don't think the comment meant that at all. I think she probably just noticed that the patio has been dirty and as her maid/helper it really is your job (yes your being paid to do a job) to clean it up. You had no right to yell at her and are lucky that she hasn't fired you. I think you should apologize for how you treated her and try not to read so much into peoples comments.