My advice is not look at this as consulting. Unless you have moderate experience surveying theaters to rent chances are you will not have the type of information they need to attract customers. Sure, as stage managers we know what we like to see when we arrive in a space. You can advise him some of the basic equipment he should keep in stock, but why specifically he can't seem to attract customers to rent out his theater? That is some in depth consulting that you may not have the experience level to provide.
If he really needs something looked over, it may not be the space itself but the rider that needs fixing. How well organized is the information about his theater and how available is it to the public? Another issue could be marketing. Does he have an updated website with current information regarding the venue and availability? Since he is attached to a high school he should take that into consideration. What kind of deal does he have with the school? Is he marketing to surrounding colleges to bring educational programs to the high school? Is he in contact with the local community theatre? This is the kind of advice he most likely needs and someone experienced in this area would be extremely useful to him. If that is you, then I would most definitely talk with him about consulting fees before going into specific details on these topics.
Since the theatre is already built, chances are there is little he can do to fundamentally change the space itself. And the best person to give him advice on equipment is someone with more than a working knowledge of the technical fields. He needs a theatre electrician or lighting designer to tell him what types of lighting equipment to invest in and a sound designer to inspect the acoustics of his space to determine where speakers should be installed and kept on hand and what type of board is necessary to run the system. A technical designer could advise on stage structure, removable aprons, traps and shop organization. As a stage manager you can tell him what non-technical equipment is necessary for the running of a show (prop tables and storage, costume quick change booths and storage, dressing rooms, booth set-up, plus general needs of monitors and intercoms, audio-visual equipment and clear-com.
Essentially, it doesn't sound like he wants a full consultant or he would be advised to get in touch with another rental house and ask for recommendations of who to call for the job. And as informed as we may be of the basics of a successful performance, without the extensive experience in those other fields I would be cautious in expecting money for what basic advice we can provide. This sounds more like a good deed. Tour the space and point out reasons why you personally would hesitate to rent the space. And if nothing seems too out of sorts, perhaps casually ask if he has had a professional look over his marketing tools and rider.
