In The Moonstone, why does Betteredge claim that neither alcohol nor sleepwalking explains the paint on the nightgown?

The paint on the nightgown turns out to be a vital clue to
the mystery of the Moonstone theft. It is initially assumed to belong to Rosanna, but
turns out to belong to Franklin, who has no memory of changing it, or being involved
with the theft in any way. When he is trying to come up with explanations, he asks
Betteredge if he had been drunk, or walked in his sleep, during that night. Betteredge
responds first that he himself had watered down the alcohol, so that Franklin could not
have been drunk, and also that Franklin has never walked in his sleep.
Furthermore:

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"The Diamond has been taken to London, since
that time. The Diamond has been pledged to Mr. Luker, since that time. Did you do those
two things, without knowing it, too? Were you drunk when I saw you off in the
pony-chaise on that Saturday evening? And did you walk in your sleep to Mr.
Luker's...?"
(Collins, The Moonstone,
gutenberg.org)

In other
words, even if one or both of those explanations were correct, they would only account
for the theft and not for the subsequent movement of the Moonstone. Betteredge correctly
assumes that someone outside of the household has been working with the Moonstone, for
an unknown purpose; neither solution explains the intervening time, during which the
Moonstone has been out of Franklin's reach. Interestingly, the real solution turns out
to be a variant on both explanations, and is discovered only near the
end.