Actually, she couldn't possibly have known him that long unless she also worked for one of his several companies in Indiana.
Part of your premise is that BM has a stable mode of being and a stable personality, but we don't know that. MG can't know that - she doesn't live with him, has only seen him in fairly recent work contexts.
If in fact he has cycles of moodiness or a mood disorder or any number of other things going on (starts things, doesn't finish, does them poorly, gets mad, takes on too much at one time, etc., etc), then perhaps other employees have eventually learned...BM isn't always organized. MG herself may not be a particularly organized person, therefore wouldn't notice right away.
At any rate, I don't think it will be easy to make this point at trial, should it come to that. I guess I'm saying that MG might make a poor witness in future, but that others closer to BM might know him better. The person who knew him best was SM and she's missing.
I of course agree that BM's failure to organize on that particular day is extremely telling, but I do wonder just how often usual things happened around him or because of him. If it's true that his "workers" were people he met casually (and who might not have been able to find lots of other work), he may have initially seemed like a good employer, but for all MG knows (or we know), BM is more erratic than he first appears. I see him as more of a fast talker than as a dedicated, organized businessman - can't say why I think that.