I reckon a red by the letter of the law, which the ref is bound to apply. I cannot argue with his decision.
However, I do think it was rather unfortunate. I really don't think he thought he was making what turned out to be a tackle. Completely unintentional.
However, this is probably irrelevant, the only prerequisite is that he challenges for the ball and it is dangerous. This means that as soon as you put your foot in that position it is strict liability, as soon as anything goes amiss you are in a position to be sent off whether intentional or not, aware of the challenge or not. Challenge plus dangerous = off and unfortunately for that lad that's what happened.