Letting A Salesperson Go
A sales person who is being fired, as a general rule, know they are doing bad and their morale/self confidence is low already.
You can actually raise their opinion of themselves if you are professional and compassionate but not SOFT when you let them go.
Most co-workers are also aware of the poor performance. You can have 2 positive impacts here 1) They see the dead weight being cut away 2) they see you are adhering to at least basic standards. The downside to not letting them go right away is the potential negative impact having this person could cause. Even the most simple cause and effect griping could have a ripple effect that would at least disrupt your operation.
As the manager it is your job to contact the clients and maintaining the situation. You should facilitate/participate in the first meeting with the new rep. Again if you do this part right it is a net gain.
We are salespeople, it is our job to turn every situation to our advantage. This can be a positive teaching moment for everybody involved. The key is not being weak or apologetic. Have and know the facts so you can speak with authority. Remember one weak link will break the chain what you are doing is for the good of the customer, company and team!