Does the agent have Dyslexia?
It is a fallacy to expect all agents to be good at both talking and emails/ web chat.
There is a fairly large population in the UK Contact Centre population that are dyslexic. They are often great on the phone but hide the fact that they struggle with written skills. This becomes most apparent if you ask them to do emails, where it is difficult to hide this.
According to the British Dyslexia Association around Ten percent (10%) of the general population are dyslexic; 4% severely so.
Dyslexia is identified as a disability as defined in the Equality Act 2010, so you may need to be careful that you are not discriminating against this agent.
The simple solution may be to ask this agent if they would prefer just to take phone calls. You may find that there are other people in the company who would prefer to do just emails and not phone calls.