Good illustrator but how good for layout?
The first question should be to ask whether he has any experience and whether he wants to do the work. Then the publisher, or self-publisher, should ask himself or herself if they have time to cope with problems.
In this case the artist was being honest, and could have been prepared to put the time in (at his cost) in finding out all the technical information he needs to know. However, there should be at least one person who has the professional knowledge of what makes a good book layout in the team.
The danger is that book layout and file preparation for printing is detailed, systematic work, and the skills for that are seldom those needed to be an artist and illustrator.
There are a few who manage to combine both skills but they are a minority and deserve every cent they can command. They will, for example, know that the uses of a book cover for thumbnails on the web or in catalogues and ads and possibly for posters as well as for the cover itself, puts severe restrictions on artistic talent.
More common are those who can take other people's illustrations and turn them into good looking and good working covers and produce files which will print without problems. The person who creates good book files and the illustrator and graphic artist may use much of the same software, but they tend to use it in vastly different ways.
The kind of questions one could ask someone who may be able to do this work could be whether he or she knows the common standards for relative margin widths, on the differences in specifications for different binding methods, and, as this is presumably an all color book, the maximum ink levels according to printing process and paper stock.
He may not know the detailed answers but if he knows what you are talking about (even if you do not) then (if there is time to go elsewhere if necessary), by all means give him a go.

