If your friend does want two catalogs, I would create two Collections, then add the Personal photos to one and Professional to the other. Once done, use "Export this Collection to a Catalog" (right-click Collection) on each, so two new catalogs are created. Always keep the original catalog until certain that everything has worked correctly.
Having said that, separating Personal and professional photos into folders/drives is an alternative and possibly a better way to go.
When @Rob_Cullen siad Remove, don't Delete, I think he might have meant remove from catalog, but don't Delete from disc.
"if the personal files are intermixed with professional in certain folders,"
Advice from @drtonyb is good and easy to understand- "create two Collections, then add the Personal photos to one and Professional to the other......" But consider that these two 'new' separate Catalogs will have lost any syncing data that the friend may have used- if SYNC is important to them.
"use another export function?"
No, NO Exporting of photos. Only the function [Export as another Catalog] is the method to use.
"He wants to maintain all of his metadata,"
That will all be in the new catalog that was exported.
"although I guess he will have to render another preview file in his new catalog that he names personal, is that right?"
No, [Export as another Catalog] automatically creates the new catalog with everything! - Previews, metadata, Collections, Keywords, etc.
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