Having enough iterations of the cycle intensive array formula is going to end up bogging his workbook down almost to a stop.
Indeed. Excel is actually the WRONG tool for this job.
By nature, a log book is much more like a database (not a spreadsheet).
With considerable banging and discomfort you can get Excel to the job, of course, but increasingly, you will come to realise that you trying to insert a square peg in a round hole.
You should be using a database, like Microsoft Access.