Your customer needs to put his foot down, hard. Dont explain the disadvantages to the rep, explain some of the following to the business owner:
Not using corporate email means:
1. Data sent and received via AOL is effectively in the public domain, the company has no control over it. Quotes, invoices, the lot.
2. When the sales rep leaves, his AOL account leaves with him. The company will have no record of correspondence between THEIR customers and THEIR sales rep.
3. Data and information exchanged is not owned by the company
4. Data and information exchanged is not backed up by the company, and probably not by the rep either.
5. Data and information exchanged is possibly not meeting legal data retention requirements, depending on which country you are in, (SOX, data protection act etc)
6. Data and information exchanged is not "corporate branded" and makes your client look like a mickey mouse organisation. It seriously negates the effectiveness of the marketing budget, irrespective of which areas of the business are being marketed.
7. Client is having a less productive sales rep (ie costing company profit) in that by using AOL he is denying himself the ability to use a smartphone to receive and process email, appointments etc
Thats a few to start with, I am sure others can add to it. Sales rep needs to grow up a bit.