1) We don't know that this person is actually seeing a "mental health doctor".
2) Seeing a "mental health doctor" and taking prescription medications are not the same thing.
How you came about this knowledge could become a problem for you in the future, depending on how it's being used.
That being said, and taking into account the other good advice given above, you could have reasonable concerns about safety.
ADA: 1630.14 of the regs -- http://edocket.access.gpo.gov/cfr_2007/julqtr/29cfr1630.14.htm
(c) Examination of employees. A covered entity may require a medical
examination (and/or inquiry) of an employee that is job-related and
consistent with business necessity. A covered entity may make inquiries
into the ability of an employee to perform job-related functions.
(1) Information obtained under paragraph (c) of this section
regarding the medical condition or history of any employee shall be
collected and maintained on separate forms and in separate medical files
and be treated as a confidential medical record, except that:
(i) Supervisors and managers may be informed regarding necessary
restrictions on the work or duties of the employee and necessary
accommodations;
(ii) First aid and safety personnel may be informed, when
appropriate, if the disability might require emergency treatment; and
(iii) Government officials investigating compliance with this part
shall be provided relevant information on request.
(2) Information obtained under paragraph (c) of this section
regarding the medical condition or history of any employee shall not be
used for any purpose inconsistent with this part.
You could make the case that you need to know if this person is fit to work around machinery if an unfit person would be a danger to themselves or others and send them out for a medical cert. Keep in mind that while we normally try to keep the ADA monster in the box, you need to take into account what will happen if you do nothing. If this person could fall into a grinder or, in an uncontrollable rage, shove someone else into a grinder, then that's something that could end up costing the Company more than dealing with the ADA requirements.
You might begin with the magic question. "Is there anything about you or your personal life that you think I should know about that could interfere with your ability to perform your job properly or safely?" If they say no, you should still consider whether or not you want to send them out for cert but you run into a problem in that eventually someone will wonder why you thought that was necessary. How you know this information will come back to haunt you if it wasn't proper and you take action on it. Proper or not, now that you know, there are civil issues that could arise out of your inaction if this person's leg gets stretched to 15 feet long when he sticks it in an auger.