It's remotely possible that somebody who answers questions on here has knowledge of the magnetic field strength of induction hobs - they might even answer your question - you can not be sure, however, that any answer is based on expertise rather than guesswork.
You will do better to ask the people who make the hob and the medical implant.
Good luck...
For what it's worth I would guess that the patient would have to lay flat across the induction hob to trigger the device - but I know nothing.
Peccavi, May 2013