//Here if field is ForeignKey i need to get related entity's value?
What do you mean when you say value? (an specific field value? what field of the related entity?
BTW, Do you know about LLBLGen v2.5 Auditing feature?
In v2.5 Auditing you can combine AuditReferenceOfRelatedEntity, AuditEntityFieldSet and AuditUpdateOfExistingEntity methods to do what you are doing... and more.