WHY IS SUSAN SURPRISED WHEN SHE SEES HENCHARD AGAIN?

She was surprised because he wasn't drinking.

"Three glasses stood at his right hand; but, to his wife's surprise, the two for wine were empty, while the third, a tumbler, was half full of water."

Further textual evidence;

"They don't fill Mr. Henchard's wine-glasses," she ventured to say to her elbow acquaintance, the old man.

"Ah, no; don't ye know him to be the celebrated abstaining worthy of that name? He scorns all tempting liquors; never touches nothing. O yes, he've strong qualities that way. I have heard tell that he sware a gospel oath in bygone times, and has bode by it ever since. So they don't press him, knowing it would be unbecoming in the face of that: for yer gospel oath is a serious thing."