I don't try to teach anyone to ride, I just offer to give them someone to ride with and come along with them. If they can't ride their own ride that's not my problem.
You may come to see this differently over time.
I should say that I don't think your choice to ride with this guy was inappropriate; a lot of us like to find others to ride with and I have had many a good time with some random rider I met out on a back road. It's an appealing thing about the sport.
On the matter of "it's not my problem," you're right in the sense that he's holding the throttle and making the decisions. Doc Wong's recent thread about a lawyer trying to sue on behalf of a rider who crashed trying to keep up with a faster rider notwithstanding, you're not legally responsible for what another vehicle operator chooses to do.
Ride with others long enough though, and you'll find some who look to you as an example and follow your lead, whether it's a good idea or not. Many years ago, a friend crashed behind me, trying to enter a corner as fast as he'd seen me do. He couldn't turn the bike quickly enough, ran off into the dirt and after struggling along in a drainage ditch and narrowly missing a power pole, he highsided at fairly low speed. Being a really skinny guy, without a lot of natural cushion, he got pretty banged up, suffering several broken ribs, punctured lung, a broken back and being knocked out.
He credited the first aid I rendered with either saving his life or at least saving him from worse harm when he regained consciousness and began fighting incoherently to stand up. While he was giving me that credit, I was feeling very badly for having sucked him into the turn, because, let's face it -- I did. While I didn't know or really expect that he would attempt to enter the corner that fast, I know without a doubt he would never have tried it if he hadn't seen me do it.
The experience forever changed how I ride with others. I size other riders up carefully and look for subtle signs that they are riding over their heads and the pace at which that begins. If I'm leading, I roll off on straights so we can stay together and I set my turn entry speeds to what I know they can handle. I watch in my mirrors to see if they are making errors and adjust the pace if necessary.
While you can't control what another rider does, you can control the "inspiration" you may provide to the guy behind you. When we are newer to riding, we don't have a real sense of how good we are or aren't and for many of us, there is a period where we compare ourselves to others through friendly, unspoken competition. Some people may have the maturity to skip this phase. I didn't. Either way, once that's over with, it becomes easier to ride with an awareness of and regard for the effects our example is having on others' behavior. Right or wrong, that effect isn't nonexistent.
when a rider fixates on something it means they end up with their bike where they were looking. Fixating On the fog line or a crashing bike= bad. Fixating on a bikes perfect line while flowing effortlessly from corner to corner=good.
Ain't it grand??? 
I learned to ride at 15,my mom's boyfriend got me a honda cb 450(1970 vintage)and took me down to the local middle school and watched me ride around the parking lot for about half an hour.
