Seems to me that the hierarchy here won't change often. This is a prime candidate for conversion to Nested Sets, which will enable some remarkably efficient queries if the table needs to be queried more than once between additions, deletions, or updates.
Hierarchies on Steroids #1: Convert an Adjacency List to Nested Sets It would also and easily allow for multiple levels, if it ever came to that.
While I usually advocate nested sets , and using industry standards like DUNS, this might be a case where his homegrown company identifier can hold the hierarchy. Basically a Dewey decimal of companies
Please post DDL and follow ANSI/ISO standards when asking for help.