Even if the kid was clearly 17, it's beyond the control of the ticket-taker. They are not the ones who sell the tickets, therefore it's not technically their problem. Once when I was about 13 maybe my family was going in ahead of me and took my adult ticket and left me with one of my sisters' child tickets. By the time I had gotten up to the gate, they had already started walking (I had a really caring family hahah) and the ticket taker looked at me and asked how old I was. Knowing I had a kid's ticket I said nine. He just kinda looked funny at me and said "pretty big for a nine year old, huh?" then let me through.
Couple things happening here. First of all, he had no problem letting in one of my sisters get through with an adult ticket. The second is I was clearly not nine years old. And I don't know, maybe because I looked like I was alone he just let me through but what can the guy really do about it? Stop the line and go get someone? Then there's an unmanned turnstile and it's a free for all. I guess he could call somebody and resume his job but what does he do with me, let me just hang out and be in the way? And what would stop me from booking it out of there? There's just too much going on to make it the ticket takers' job to spot 'liars'.
Another thing is that it's simply above their pay-grade. Minimum wage hourly workers shouldn't have to spot out ticket fraud. It's not their job to judge families or call anyone out on being dishonest. They don't even have financial stake in it. In the end, the system is about honesty, and Disney relies on that and their guests being good people, which I think is reasonable. The reason the ticket taker probably questioned the woman was because he could get some kind of reward for spotting fakers but again, there's nothing he can really 'do' about it, besides get her to confess. Disney also relies on people being scared that something will happen and that some ingenius system is in place to nab the liars, forcing them to tell the truth.
In any case, this probably rarely happens and at the insane prices they're charging now, did they really lose that much from this kid?