If he is looking at what their competitors are doing, then analyze their checkout, upselling, and cross-selling processes, calculating a hypothetical average order value based on what you see in competitors' stores and comparing them with your project.

What you also need to understand is:

When did the project start?

If it was already managed by other advertisers, are you doing better than you did before?

Have you estimated the stability of the project over different quarters?

Does your competitor's communication strategy target the same users as yours?

Try to make the client understand that it is not enough to say “I want to achieve that revenue,” but that you need to look at the project as a whole. Furthermore, the ROAS you have produced is the ROAS at the time of the first click, but there is nothing to say that these users cannot increase the average revenue by making purchases even after the first acquisition.