No, it's not like a mass in her lung, it's like an air ballon that is on the outside of the lung, instead of inside, thus deflating the lung in that section. I have never heard of excessive exercise causing this, but then again, I don't claim to everything.
The easiest way I can think of to explain it is that the lung actually "sticks" to the pleural cavity lining. This sticking allows the lungs to expand when we inhale and deflate when we exhale. In your friend's case, there's a small section where the lung isn't "sticking", thus is not expanding with the rest of the lung. The resulting air pocket or fluid buildup (because the body does not like an 'empty' space and is trying to fill it up) prevents that section of the lung to re-expand. Overdoing it can cause it to get worse, but if necessary, they can empty out that air or fluid pocket to encourage the lung to re-expand.
Clear as mud?;-)