Beginner advice needed [HELP]

Hi folks!

So the problem is i got a few costumers ( ngl 1 costumer is my brother) who wants to optimize SEO for their website but the issue is that they only have 1 page website (kinda portfolio/cv web) promoting their service/product.

Soooo.. i know that having only 1 page is a limiting factor but with this types of sites i cant figure out how indexing more pages will make sense ( as it isnt like a blog kind of web).

What are ur ideas on what should i do/implement guys??