I've never bought a round for these reasons - hard to know if the jobs are priced right, and you might well buy messers. Back to OP's point.
Go round and see if she has a valid point - are they dirty. If they are dirty re-clean them and apologise. If not then explain that they are clean in your opinion. If she is talking about spider's nests in the reveal on upper (or even out of hand reach ground storey) windows, explain that wfp will clean the frames, and "to a greater or lesser extent the reveals". WFP is perfectly good on first cleans, with the right technique (TFR, time, flow rate, brush head).
If you get them sparkling and she is happy post-visit explain you are glad that she's called you back as it gives you an opportunity to explain the system; why wfp method delivers a better clean (in my opinion it does, but that's not for here) and prices have to rise as the other windie had underpriced the job. Explain the new price is £13 or £13. Ask if she feels she needs a monthly clean (does she?). Could she benefit from going 8 weekly - little difference in how clean the windows are if she's in a quiet cul de sac - but monthly cost still same.
If she looks like she's been slapped with Tosh's todger then you could then offer to try and find another windie but regretfully you won't be able to continue to provide a service. 3 months in and you'll still be canvassing so should be easy enough to replace her, especially given how much she contributes to annual turnover...
I rarely get callbacks, I think I've had 4 in all. 1 was a poor clean and I don't know why; 1 was a loony; 2 the marks were on the inside and the outside was perfect.
Be fair - address complaints, price fair, dump if necessary, do a good clean every time and you'll have a good business.