DJT,Jr tweeted last night that Daines told him he "had it covered," inference being he was going to vote. How he actually does that, don't know.
Procedurally, they could hold the vote open for him to return. Or, he could use "vote pairing," and make an agreement with a Dem that the Dem won't vote either. That would cancel out his absence/non-vote.
(personally, at this point, I wouldn't trust any Dem to actually live up to a vote-pairing agreement.)
https://www.washingtonpost.com/powerpost/final-vote-on-kavanaugh-could-be-delayed-for-hours-as-gop-waits-for-daines/2018/10/05/d450fbaa-c8c2-11e8-9b1c-a90f1daae309_story.html
If Senate confirmation of Supreme Court nominee Brett M. Kavanaugh comes down to a single vote, Sen. Steve Daines of Montana could keep the world waiting for hours on Saturday.
There is no doubt that Daines, a Republican, will vote to confirm Kavanaugh when the final vote is called, but his personal calendar is a complication: His elder daughter, Annie, is getting married Saturday in Montana.
“At the end of this weekend, I will have walked my daughter down the aisle at her wedding and there will be a new Supreme Court justice,” Daines told reporters Friday. “We’ll wait and see how this all unfolds here.”
The most likely outcome, according to a senior GOP aide familiar with the discussions, is that the vote will be held open — potentially for hours — while Daines flies back east.