I wouldn't have any problem with the other guy doing the fascias, but I would be a little miffed if I were to be told that he would be doing the windows in my place, then to be told to start again the following time. I always respect the customers' right to choose, but I want to be either their window cleaner or not their window cleaner.
I wouldn't show my feelings though. If I were to say anything, it would be along the lines that I wouldn't want to take the other guy's work from him, so why not stick with him, all communicated politely and with a cheery smile.
I had one recently. I clean this house occasionally, when it suits her.
She had asked me to call to her the next time I was around. When the time came to call, she told me she had got her brother-in-law to do them. No problem, said I.
I am due in her area now, and if she asks I will use the line I mentioned above.
John