That a huge curve ball. The timber scaffolding will be treated as temporary works and needs to be designed. There are design codes for timber structures but you will need a structural engineer to design it. They will also inspect it for you. I would contact a local practice and for a simple structure the fees will should not be much. However there are number of practical issues. Structural timber has to be graded and is usually marked. There is statutory inspection for scaffolding. It may be cheaper to employ a scaffolding company. Whether the monastery can be classed as a place of work is dependent on how they govern themselves. They may be a non for profit social enterprise and be a registered company or partnership. If they are it is a place of work. You could check to see if they have employer’s liability insurance.

What order are they? Is this a religious question? Is it an act of faith that they use a wooden structure? If no, then ask are they going to use it to do a job of work? If yes, then advice them hire someone or something is the best option I'd have thought.