It seems to me that IM was trying to discredit Erin, or make her look bad. Because IM not only said Erin was supposed to start at TSC, but didn't; but IM also said that bit about how none of the family checked on or inquired about the horse, even though Erin had already told IM she couldn't take the horse. So that begs the question: why did IM want to make Erin look bad? All of this is MOO.
That is how I took it, too. Actually that was my first thought. My second was that IM wanted to seem like she knew more than she really did. I didn't get the feeling she liked Erin very much. I don't mean to imply that she was involved in what happened to Erin in any way. I just get the feeling there wasn't a very close relationship/friendship there.