We have an individual who has been off the last few weeks with stress. She says that this is the result of dealing with an individual employed by one of our major customers with whom she is required to liaise on a regular basis. Her latest sick note expires at the end of next week. She has been with the organisation for a year.

We have spoken with the individual employed by our competitor and he feels that our employee is not up to the job. Comments from her colleagues appear to confirm this.
While she has been away, another individual has been filling the role. She has performed the role well and has no problems dealing with the customer in question.
The manager now wants to start capability proceedings against the employee. I am nervous about doing so while she is away with stress as I don't want the organisation to be seen as exacerbating her condition.

Does anyone have any advice?