I would be surprised if he was completely out of it, but I figure he will have his moments no matter what.
I think Trump should prepare as if he is facing a good debater. Even if Biden is pathetic, Trump needs to look like he deserves people's vote.
If there are any drugs which can improve cognition - even for a few hours - believe me, Biden will be on them.
The WORST thing Trump could do is assume Biden will be way off his game when he shows up for the debates - Trump should practice against a "good" debater (Ted Cruz?) and prepare as if he were the underdog.
Which in a way, he is - for one of the debates, the "debate commission" chose A FORMER BIDEN INTERN to be be the moderator! So he'll be facing a two-man team.