I think if 1) she wasn't so talented in her own right, and he had to defend someone with no credentials, he'd be crucified; 2) if things went south between them and she claimed she was NOT hired due to a relationship, he'd be crucified; 3) if she either was or wasn't hired, but they said the relationship began beforehand, he'd be crucified; and 4) if there was someone else qualified (or not) for the job who was claiming they should have been hired, he *might* be crucified.
But given the circumstances, and given that its not the kind of job that is open to all takers to begin with, I doubt claims of cronyism (or nepotism) would go any where unless there were a complaint.
It's just hard to get worked up about a 22 or 23 year old dating a 35 year old, in the same field. And its hard to get worked up about the young person being hired on in any capacity when they've personally been so successful in that field.
A good rule of thumb is: would she have been hired were she NOT dating the coach? While we can't know, the answer is probably.