Isn't it against employee's rights to be terminated because of a disability?
It is illegal to terminate an employee BECAUSE he has a disability that makes him eligible for protection under the ADA.
It is not illegal to terminate an employee who may be collecting disability benefits but has been unable to return to work after the term of medical leave provided by the employer. Not even the ADA offers unlimited medical leave.
Only a fraction of situations for which an employee can collect STD benefits offer ADA protection. With the sole exception of HIV/AIDS, every case, without exception, has to be looked at on a case by case basis.
Multiple courts have ruled that an employer is not liable for accomodating a condition under the ADA that they have not been notified of. Collecting STD benefits is NOT notification since not all situations for which STD benefits might apply, are ADA eligible.
With all of the above, more information is needed before it could be concluded that a wrongful termination has taken place.

