Access to Edit only a particular field in a record in CRM 2013

Hi,

I have a form which has 10 fields in it. One of them is a lookup of all users in CRM and there is a corresponding dropdown field of yes\no. The user with read\write privileges on this entity can put any name A in the lookup field. A has read only privileges for this entity but he needs access to edit only the dropdown field and not any other remaining 9 fields in the record. Kindly let me know how this can be achieved.

Please note that the lookup field will only contain names who have been assigned a particular role. 

Thanks in advance