@browncoat - There might be some truth to it, but she would have to be very careful not to hurt herself by getting too dehydrated or being exposed to too much sun.
There are definitely athletes training at deliberately high altitudes or hot conditions in order to condition their bodies to the point where easier conditions don't feel as difficult.
I'm not sure if casual runners really need to do this though. But I know that runners can be very competitive, even just within their own friendship groups. My best friend does a lot of running and she's always joking about wanting to beat the other people in her running group, even though they are quite supportive of each other.
I think it's very important to stay within reason though, as it's easy to get caught up in a culture of competition and go too far with your health.