Transmission Overheated

I borrowed a friend's F250 with the same motor and trans to tow my 38ft 5th wheel when my truck was out of commission. I ended up in stop and go traffic in 85 degree heat and after about a half hour I started to smell hot anti freeze and saw that the temp gauge was way too high. I pulled off into a parking lot and when I shut down the engine not only was the anti-freeze boiling out but there was transmission fluid boiling out. I let everything cool down and put in a couple of quarts of fluid in the tranny. Some anti freeze to get the levels up and headed down the road. The traffic had cleared. Everything was great, no more problems with overheating or boil over. MY friend has never had the problem since.