Columbians are a bit different then the others. Aside from looking, eating and acting different--most are imported. If this breeder captive bred this lizard he is in the minority because Columbia readily exports these guys very inexpensively.
They are not as cold tolerant as the tegus from argentina either....and are not as hardy.
When you put it all together it could have been anything (basking temps definately needed to be higher).
If you invest in a tegu in the future, for so many reasons, try and buy an Argentine Red or BW. I would go with the BW...Columbians are problems waiting to occur. They are nippy and then as they get older they begin to BITE 90% of the time. Neither of my tegus were nippy as babies. Columbian tegus are very poor captives, and are not pet quality animals at all

Very sorry for your loss though. It's hard not to feel terrible when these things happens but it doesn't sound like this is your fault.