If it is a silicone based sealant you have a real job on your hands, I pretty sure (though not certain) that cellulose thinners won't help, and by and large, cellulose thinners won't touch ordinary paint.
I would personally tell the customer I can't get it off the frames without charging an absolute fortune, if they are really covered you could be spending well over 30 minutes per window to clean the frames properly.
This is the decorators problem, she has now paid him up in full of course and can't get hold of him, so now she is stuck with sorting it out herself...
I wouldn't touch this job with a bargepole!
Ian