I think many people on the left figure Kavanaugh won't be able to effectively refute this. After all who can say where they were 36 years ago. Obviously, it is tough to have an alibi when the accusations are so vague regarding time and place. The Senate will be trying to pin her down on that, and the more vague her answers, the more difficult to refute. But also the less credible they will seem. You were 15. Was it summer? Beginning of summmer? End of summer? etc. etc.
But after thinking about it, the fact that he was still in high school may help him out. Did he even drink when he was 17? Had he ever been drunk? I hadn't (yeah, yeah, late bloomer) Athletics were important to me, and I had a few drinks during the summer between my junior and senior year, then nothing until the spring. I had one friend who was a straight arrow who never drank at all until he went to UT. Yes, it may be difficult to prove absolutely, but he may have some strong circumstantial evidence that will be believable. Her story is that he was drunk. I could effectively refute that. He may be able to as well. He was also from a wealthy family. It's very possible he was gone most of the summer, and can prove it. I frequently was, even if it was considerably less glamorous than vacationing in Europe.
And given, the way Republicans have bent over backwards to get her testimony, when she gave them reason to just shut her down, I have to believe the Republicans believe Kavanaugh will make a good showing. I still think she finds an excuse to not testify, but we will see.