Don’t. Run your race based on you, not based on how he’s doing. It’s hard to race individuals head-to-head in any triathlon, but especially a 70.3 or longer where efficiency and plan execution matter so much. If you get wrapped around the axle of trying to put more time into him on the bike, or pushing harder on the run to keep your lead, you may blow up completely. I’d recommend running your race plan focusing solely on that plan, and then when mile 10 or 11 comes on the run, you can start worrying about head-to-head stuff if he’s close.
Have a plan. Execute the plan. Don’t worry about racing until very late.
#protip: take advantage of “free time” by dialing in your transitions. In a situation where you expect to be within a minute or two of someone, transitions can be the difference. Many longer course athletes don’t worry about transitions, so if your buddy is one of those guys, put some time into him in T1 and T2.
I wrote out my long-winded transition tips for sprint tris here.
For longer events, I do the same things, but obviously nutrition is different. I usually take a gel in T1 in a 70.3, usually a caffeinated one. I also usually have a bottle of water there to wash it down with a quick swig… That said, the gel is open (or in a bottle) and the whole thing takes about 5 seconds. I also wear socks in 70.3 and longer. The socks are rolled to halfway down the foot so literally all I have to do is get my toes into the ends and flip the socks over my heel. (helps to practice).
If you get T1 and T2 dialed in, that can gain you seconds or even minutes on him for no more effort.
Now, if it comes down to head to head on the run, you’ve got to try to break him one way or the other… for most people it’s easier to sit on his shoulder and make him dictate the pace while you just hang on until he dies or you make the decisive move on your own timeline. That said, odds are he’s going to sit on your shoulder since he’ll come from behind, or just try to blow past you.
If he tries to blow past you, get on his hip and stay there. He may exhaust himself trying to drop you, and if he does drop you, odds are, it’s over unless he blows up anyway. So stay there as long as you can and make your move late.
If he sits on your shoulder, you’re in the other situation: you have to try to plant that seed of doubt in his mind by pushing your own pace. Get him to the point where he’s questioning if he can maintain the pace, and you may find that he drops off. If you can gap him again after he catches you, you can probably beat him.
Again, both of those are reserved for late race. You can’t go trying to blow someone up at the 5 mile mark of the run because odds are you’re going to blow yourself up with plenty of time left for them to catch you.