In my opinion, if the frame was correctly spaced to 135 and now it actually measures 142 inside-to-inside, there is either structural damage somewhere, or your weight is actually causing the stays to spread. (No insult meant, just an observation on physics). It's unlikely that the fellow who did the coldsetting could have done anything to cause the problem, but it's always a good idea to check the brazing/welding around the seat and chainstay bridges after such work. On rare occasions the area can be damaged by the process, and perhaps even stressed but not broken, with visually apparent damage showing up down the road.

Regardless, in you shoes I would very carefully inspect the brazing/welding at the bottom bracket shell where the chainstays enter, the chainstay and seatstay bridges, and the dropouts. Even faint lines in the paint may indicate cracking and would need to be checked and possibly repaired. If absolutely no damage is found - and you're absolutely sure that the hub is 135 and the inside-to-inside dropout measurement is 142, I would simply squeeze the dropouts back together by hand and then pay extra attention to the spacing over the following months. If the problem recurs, the frame most likely just isn't strong enough for you and should be replaced before anything catastrophic happens while riding it.

Good luck!