I'm still trying to get my head around this, it's certainly a very strange break, to which I have a theory.. But first, you say it cracked a few months ago, but still continued to clean it ? Why? And did you not inform the customer at the time ?
Also in your picture to me there's water droplets over the glass and on the underside of the frame which appears to me that you have just finish washing it - is that the case ?
This is an upper window, then all things being equal you are pretty much the only ones working on the glass, unless a 12ft tall giant happens to pass by- so it's very reasonable to assume you may have damaged it, ground floor very different of course.
I would say that the brush head mat have been slammed onto the glass to hard ( purely by accident ) and maybe more than once.
You guys are the only ones to know what's really happened here, the fact is there are 2 of you, so in that respect you have a strength in numbers as to being accused of any wrongdoing, any cracked or broken windows should be noted/photographed and I wouldn't be cleaning any part of that window ( better left dirty and untouched than cleaned to cause confusion )
Over the years I've had several windows that were broken on arrival, and always done the above, and it's never gone any further, sometimes the customer is already aware but always grateful to be informed.
Also had a guy work for me who broke a Georgian window, he claimed it wasn't him and JUST after he had cleaned it a sparrow flew right into the glass, smashed it, a flew off
Strange how there was a spray of water inside on the floor and on a settee - so should I argue to the customer this mystery sparrow caused the damage or just get a replacement organised, of course I got a replacement sorted, the customer got a free clean to. ( that was more than the glass!) but four years later I still clean the property and several of their friends.
Sorry, think I waffled a bit there
Still I'd be interested in your answers to the q's
Darran