Sorry OP, I know it's upsetting that they died - we are all upset right along with you and your mom. You said your mom said it was "VERY hot in the coop". So while we have no idea what the particular situation was with your mom's coop, we can at least gather that it was "VERY hot". That's probably too hot.
When it's really hot, it's important to do something to bring the temps down (coop temps and/or body temps). There are articles which explain that chickens actually begin to more or less 'cook' at 90 degrees, because they cannot sweat to cool down in the way that we do. The situation can be very urgent at that temp or higher, depending upon the breed, individual constitution and the blasted humidity. This is why fans, ice, cold fruits, temporary relocation to cooler area when possible, can be crucial to avoid death. If all of those things were done and they still died it's one thing but if their basic needs aren't understood and met it's just all the more sad. Don't know what the situation was in this case - the only thing we know is the description of "VERY hot". Maybe it was that hot despite all manner of cooling actions having been taken - don't know.
I'm so very sorry for the loss of your mom's birds.
JJ