What is the nature of the back pain? A lot of people have imbalances, such as tight hamstrings and week abs, that can lead to some back pain. That would be different than something like, a disc issue, or spondylolisthesis. My first piece of advice to a student is always to confer with their health practitioner if there is an underlying structural issue, or an injury.
My issue generally would probably be less about the heat per se, than that most varieties of hot yoga are relatively physically demanding. Depending on the nature of the cause of the pain, they may be able to do it, or they may be better served with a more yin practice.
There is also the question of whether the person is a practiced yogi or yogini, which can make a difference. An experienced person often understands how to be mindful to necessary modifications, and knows how to make them.
Sorry not to have a more simple answer.