You need to address the attendance and performance issues WITHOUT making any assumptions on drug usage. If you make assumptions, that could inadvertently bring this person under ADA when they normally would not be. Discipline her for attendance and performanace and let that lead to her termination. Under ADA, it is the EMPLOYEE's responsibility to broach the topic and accommodations.
From : http://www.eeoc.gov/facts/ada17.html
"Anyone who is currently using drugs illegally is not protected by the ADA and may be denied employment or fired on the basis of such use. The ADA does not prevent employers from testing applicants or employees for current illegal drug use, or from making employment decisions based on verifiable results. A test for the illegal use of drugs is not considered a medical examination under the ADA; therefore, it is not a prohibited pre-employment medical examination and you will not have to show that the administration of the test is job related and consistent with business necessity. The ADA does not encourage, authorize or prohibit drug tests."
It goes on to say later in the same article:
"Q. Under the ADA, can an employer refuse to hire an individual or fire a current employee who uses drugs illegally?
A. Yes. Individuals who currently use drugs illegally are specifically excluded from the ADA's protections. However, the ADA does not exclude:
- persons who have successfully completed or are currently in a rehabilitation program and are no longer illegally using drugs, and
- persons erroneously regarded as engaging in the illegal use of drugs. "