There’s a lot going on in this situation and several clarifying questions initially come to mind when reading this scenario…. Assuming here that Sue reports directly to you Profe

There’s a lot going on in this situation and several clarifying questions initially come to mind when reading this scenario….
Assuming here that Sue reports directly to you Professor Kelsey. I’ll also assume that you have not yet discussed any of the unsatisfactory performance related issues directly with Sue, due to your travel schedule.
In addition, I’ll qualify my comments a bit upfront: Prior to taking any formal action, a decision should be made around whether or not the firm believes Sue can turn around her performance or if this is an unfixable skill issue. Concerted effort should be given to determine this due to the potential impact on the person’s life. Ideally this would take time to fully understand the situation and the path forward. With that said, I’m basing my recommendations solely on the information provided.
While Sue allegedly has 15 years of secretarial experience, there appears to be deficiencies in her ability to complete the minimum job requirements outlined in her specific responsibilities. In addition, it appears that her ability to learn new tasks is limited, as well as her critical thinking skills. Furthermore, the quality of her work is negatively impacting the productivity and morale of the other employees. Based on the information provided as is, this appears to be a bad hire and one that is not a good fit for the firm. With that, my recommendation would be to coach Sue out and replace her.
First step is a verbal counseling conversation directly with Sue to address the performance related issues. Given that you have not yet ever discussed any performance related items with Sue, this may complicate the matter a bit and lengthen the “coaching out” process. Ideally you would have been proactively coaching her performance along the way and providing timely feedback…..but given your travel schedule, providing timely feedback was probably difficult. Prior to your performance conversation with Sue, gather facts and document examples of unsatisfactory work. This will allow you to stick to the facts during your discussion and provide specific examples of where her work fell below expectations. Redeliver her the job requirements and expectations, and confirm whether or not she believes she is capable of meeting the expectations. Document the details of this discussion.
(Assuming the poor performance continues for another 30-60 days), the second step will be to provide her written counseling of her underperformance and place her on a formal 90-day performance improvement plan (PIP). The discussion will be similar to the prior verbal counseling conversation. You’ll need to document clear expectations and performance benchmarks that will need to be achieved within the performance improvement plan timeframe. Upon being formally placed on a PIP, it is common for folks to seek alternative employment elsewhere. However, if Sue has not self-selected by the completion date of the PIP and has failed to hit the established performance benchmarks (which is assumed here), you will be able to terminate her employment for cause.

Additional recommendations for the firm:
I recommend that the hiring process be re-evaluated. If Sue was selected out of 100 applicants and was not the right fit, there may be some gaps or areas of opportunity within the hiring process. Try to determine why this failure happened. Consider including a broader group of people in the hiring process to more adequately examine the candidates for the right fit, expand the line of questioning to gather better evidence of core competencies required for the role, provide an employment referral bonus and seek employment referrals from current employees that know the workplace culture, require and check references from prior employment, etc.
In addition, I recommend for some other employee be the designated supervisor of the new administrative assistant. Someone who has the time and capacity to proactively coach performance and provide timely feedback.
Comment above discussion in about ten or fifteen sentences.