There does have to be a record of the opinion being sent back, unless the two providers share the same medical record. So a copy of the 'opinion' letter needs to be kept in the chart if the provider who requested the consultation is from an outside practice.
But one thing to keep in mind: a referral is not a consultation. A consultation is a specific request for an opinion from a specialist. When a provider requests a consultation, they are asking for another provider's input - they want that provider's opinion in order to assist them in diagnosing the patient and/or creating their own treatment plan. It's different from a referral which is simply sending the patient to the specialist. I've found it helpful to explain it to providers this way - they shouldn't be consider a visit a consultation just because a patient was referred to their practice.
If the patient is just being told, for example, 'you should see a dermatologist about this, here is the name of one I recommend' - that is just a referral. But the the provider is sending the patient because that provider needs to work together with the specialist , for example a surgeon is planning a procedure and the patient has a heart condition that could be affected by the surgery, the surgeon may need the cardiologist's opinion on the risks or the right way to manage the heart medications before and after surgery - then that may qualify as a consultation.