To be fair you can't cure stupid and nor is it really your responsibility. What kind of idiot does such a stupid thing ?. Turps or white spirits may help but if that was me, I'd advise him best I can and just do glass only until he has resolved the issue, after all you'll just be taking his paint flecks onto the next customer.
Hi if you going to help him I dont think white spirit or turps will Work , Methylated spirit I think you need and bit Of Rubbing but don't hold me to it , see what some other guys say on here if they Agree, if you are to clean the windows , just do the windows stay away from the frames for now , or you will be blamed when they don't look great due to any Disclouration from the frames .
If he has painted over a dark wood effect pvc then I would let him crack on himself any chemicals used to get the paint off could damage the wood effect wrapping over the frame as it's not a solid colour and they could end up looking an absolute mess and may never get all the paint off he will just have accept his mistake and live with it or replace the windows.
I once used do a house and husband decided to paint the wooden leaded frames he under coated but never got round to glossing them. I was tradding at the time and warned that the glass would probably leave a " haze " when they have been scrimmed over he said no problem and after the second clean he sacked me and told other people on the street what a **** job I had done ?.
