This is highly suspect hypothesis. If there were/are any relevant scientific studies, then we would have a basis to begin to analyze the veracity of this ...idea.Here's one link that tells you about the dangers of bottled water in a hot car or being frozen....
Snopes says its false, but I still wont take the chance. they have been wrong before!
People drink hot water from a car, from time to time and get cancer?
People smoke cigarettes obsessively and might get cancer...after 20+ years mostly.
Perhaps its not the safest thing, but its so unlikely that anyone could get breast cancer from this type of situation! Plus there are way too many variables to isolate "hot water" + plastic = "breast cancer"
Hell I've drank this type of water and smoked out of plastic several times years ago.. *bad idea* But I'm fit as a fiddler (not that this is any kind of proof..i'm just saying)