Did the auditor actually quote a non-compliance, or did he just suggest the corrective action (which is another no-no)?
Why is it a "no-no" for an internal auditor to be involved in the corrective action?
I'm not advocating the auditor be responsible for fixing the problem, but who is better suited to help when a department needs it? The auditor has an opinion of what they "should have seen", and probably has experience in what "other areas of the company are doing". There is no "consulting" rule for internal auditors that I am aware of. For "value for your buck", the involvement of the auditor could mean less time spent in trying to figure out "what the heck do they want" and "would this even be appropriate". This translates to fewer dollars spent and which company won't take that?
Another way to look at it - once the audit is over and the non-conformance is written, the internal auditor goes back to work as whatever the auditor normally does. A simple assignment to a CA team would place them right back in it anyway. Why play games? Use the best resources you have to get the job done.
I hope I have thrown the cat far enough into the pigeons.