Suki was in the vicinity when Toph went to get Sokka. She followed them back, and she's been watching events with growing dismay ever since then.

Now that she knows the full extent of Katara's 'problem', she feels pretty terrible for not telling anyone what she'd gathered before. Fortunately, her vantage point is a little ways from the main group, and from Mel, so her guilt shouldn't be apparent to anyone at this point... unless they can sense it through vibrations in the earth.