(Don't answer, we can come back to it later if it's true, the last chapter probably)
I hope it's not because she's a traitor and actually because she has another very important task given by the parents like investigating X person or assassinating the big villain dude, so the help from the highly skilled foreigner was a welcome one. If she ends up having to kill the maid I'll cry
If she's a secret traitor she could've let things play out on the train and nobody will suspect anything.
Unless her abilities aren't a secret to someone else out there who isn't going to die any time soon, so she knows she wouldn't get away with it (but who cares if girl is dead).
It's also not intentional sabotage by hiring a random bodyguard betting they would naturally fail. Under the same logic as above.
So I'm guessing she had to do it right there and then because she knows something about our main girl and her coincidental appearance surprised her and that is important enough to swap plan on the spot to keep her close.
E: Rereading chapter 1 I don't think that was hinted at all. It does seems like an on the spot bet.