I raise trout in a smaller 1/10th acre pond. Brooks, Browns, and Rainbows. I run up to 45 gpms of aerated well water into the pond 24/7 in summer.

The answer to your question is it depends on what temps are in the deepest water, and if D.O. levels don't drop below 5 ppms in the that water if you allow the pond to stratify.

As I have said before on this site: Two things you can do:

1.)Check temps from top to bottom during the hottest part of summer (sounds like you just checked the surface).

2.) Or put some trout in before summer and see if they make it. If they don't make it...well... there's your answer.

Optimum temps for trout is mid 50's although they can acclimate to the the mid 60's especially rainbows although you will probably lose some to disease.

Your friend could also divert the spring water into a much smaller pond (easier to cool) and then allow the overflow to go into the 1/4 acre pond for coolwater fish.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.