if she heart broken, it will affect her career and your job too. plus, teenager are more emotional and it could spiral to worse case scenario.
the only things he could do is that maintain certain distance that close enough that make her feel secure with her relationship with him. remember, he not grooming her, its her grooming him. she knows that he can't continue reject her advance and keep his career. she know that he is kind enough to not let her unhappy so she taking that advantage.
this might be controversial takes, but if she grew up as adult and still have feeling, he should take her. he not actually grooming her to have her as lover. its her waiting herself to grew up so she can date him legally. i think to consider grooming, you must have a intend first and actually do emotional manipulation to mold the target to attach to you.