Can anyone help with some advice, please, as this is one I have never before experienced?

A team leader in a Govt organisation, has inherited a new team within the last 3 months. Within the team are a number of difficult characters, with whom she is coping and managing towards shared goals and objectives. However, one member of the team is extremely disruptive. She repeatedly and agressively challenges the manager in front of the rest of the team. The team members sickness record isn’t great, but some of it is DDA qualifying. The manager is therefore very careful with any feedback, so that it doesn’t set the employee back on the sick. That aside, the team member is actually not very good at the job. Her quality is poor and therefore the manager has to explain repeatedly how to do tasks according to policy & procedure. This is done in a calm and patient manner and not at all agressively. However, each time the manager has to raise an issue with this team member, the team member gets agitated and phones the union to discuss, threatens to bring a grievance against the manager etc etc. Whilst more senior managers are aware of the situation, very little support is being provided to the team leader, other than reassurance that she is doing the right thing. However, the employees behaviour is such that the team leader is now herself becoming ill. She wants to raise a grievance against the employee but is being advised against this.

What advice do you have for the team leader on how to proceed?