My friend's brother broke into someones house. I say to my friend:
He has to understand that what he did was/is illegal, it's called trespassing, and the cops can take him in for it/that.
Are both okay here?
Is works for me too.
Why is it that both are okay for you?
He has already committed the illegal act. When he did it, the act was illegal. The act is still illegal so the present tense could be used just as easily.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.