Bad handwriting could be a sign of dyspraxia. This could mean that this person also has other difficulties with coordination, spatial awareness and a sense of direction.
Fine control of small instruments like a pen is a typical problem for someone with coordination disorders.
Avoid giving this person tasks that require very fine motor control. Don't ask them to write by hand and create very clear and concise forms for them to fill out.
Try to provide an uncluttered work environment.