For those for whom the notion of the consumable type is not very clear in Inventory. 

Into Odoo, a consumable is a physical product which is treated like a stockable product, with the exception that stock management is not taken into account by the system. You could buy it, deliver it or produce it but Odoo will always assume that there is enough of it in stock.

Examples of consumable products: screws, nails, wrapping paper, salt, ... 

As you do not manage the physical stock of consumable products, there is no possibility to set up reordering rules but you still need to know when to replenish. An idea for your customer is to use the Kanban process.

Here is the interesting link to an article in French (sorry) which explains the principle quite well. 
https://www.manutan.fr/blog/methode-travail/gestion-stock-methode-kanban/

I have translated below the most important part: 

The Japanese Kanban method, very often used in a Lean environment, is proving to be an efficient method of stock management. With it, you only fill up your stock and replenish it when necessary, in this case when a visual indicator called Kanban (a cardboard card that you attach to the bins or parts containers) appears. 

There are two main methods used for Kanban: 

You have at least two bins per product. The first bin, placed at the front, is the operational stock for daily use. In the second bin, at the back, the reserve stock is stored. As soon as the first bin is empty, you know that the stock needs to be replenished. The second tray then slides forward.

With black adhesive tape you mark the locations where the product is located. In the middle of the slot, you mark a line with adhesive of another colour (such as red). As soon as the red line is visible, it is the Kanban signal that the stock must be replenished. The items in the front bin are sold out, but you still have enough stock to work with until replenishment arrives. At each bin, you hang a kanban card with the minimum and maximum number of materials that it should contain.


In attachment, you can find pictures to illustrate the kanban method.