I don’t believe that an employee’s salary may decrease but it’s more on the rate will increase especially if you are doing excellent in your job title. Maybe you are trying to say that his salary now is likely struggling to cover all the bills and expenses due to rising prices of goods and services which I think is understandable due to rising inflation. Well, your friend is not alone, as I bet everyone here has experienced that. Having a side job is the key so that if your main job salary is not enough, you still have another job that you’ll expect to pay you.
People forget that when a countries economy is bad, it affect almost everything in that country and the prices of good go up making it difficult for one to be able to afford a living like they normally do when the economy was stable, maybe the economic situation in his country is bad and it's very obvious that his salary is struggling to cover up bills and expenses like you mentioned.
For instance the economic situation in my country have become so poor that most times an average citizen with just only one source of income could barely afford three square meal on a daily basis, the prices of foodstuff have trippled and therefore if one is not earning extra money or have a side hustle to cover up expenses then they'll be thinking just like the Op's friend that their salary is reducing.
But then having a side hustle is not a bad idea, I believe that's the best solution and what the OP should've suggested for his friend, atleast it would help cover up his expenses until things get better and even save for the future, normally i don't even expect anyone to depend on only one source of income, I bet if he had other sources of income he won't had even noticed that his salary is not enough to cover up his expenses.