Sorry for your loss. Always sad and hard to accept the death of any animal especially those in our care. I noticed you said you thawed the mouse but didn't get how you warmed it up for the snake? Also i would look to husbandry possibilities before considering IBD as a cause just bc IBD is usually a long shot. How were you heating the enclosure is very important and the regulation of the heating? These snakes need temps in the low 90's to properly digest and thermoregulate.