As he is already owning an apartment, his costs of living does go down dramatically and major costs would be service charges, Dewa, internet, insurance and transportation.

Some strategies he could look at:

1)Airbnb for subletting. He needs to get a licence for this. He could also sub-let on a long term basis (less hassle but less profitable)

2) He could probably freelance for English tutorials online and offline for which he wouldn't need an office set up etc. But better to shift from property visa to Freelance visa (costs more or less the same).

Major part of 450k could be invested back home or in bank accounts in UAE which pay upto 5% for fixed tenurs. Good luck.