[TX] Tenant unfairly treating my friend!

hello folks,

My friend is in a bit of pickle here, and was just wondering if we could get some recourse.

My friend has been been paying rent on time at his store for 20 years now. His next door neighbor has not been paying rent for over 2 years now, and is behind well over $50K in debt to the landlord. My friend is trying to haggle for a better deal, but the landlord isn't budging. He's paying his rent on time every month but in increments rather than full because business has slowed down.

Is there a fair claim for discrimination here, or is he SOL?

Thanks for your help.