I know nothing of the particular case, but this argument isn't very convincing to me; the existence of a disparity does no work to show that disparity was actually part of the decision making process for the 17 year old in the way you're insinuating. The more we start removing the agency of late adolescents by calling them 'impressionable', the less sensible this conversation becomes, as though nobody can remember when they themselves were once 17, I know I can.

The argument is flawed because we generally tolerate power differences in relationships: men and women (institutional power), certain cases of coworkers of different ranks, an able-bodied person with a disabled person, a depressed person with a mentally healthy person, a suicidal person with a happy person, etc.

There are also a lot of adults who vary, due to personal characteristics or experiences with relationships (or lack thereof) who are far more easily snared into a relationship; the average 17 year old has had many more relationships than I have as a 25 year old, and sexual ones too, usually with peers. If anything, I'm the naive one who would fall into a very convincing snare with an older man.