Short answer: The passive voice with the simple present with “to decrease” may be “correct”, but it’s probably not the best way to put it, depending on what we want to express.
“To decrease” can certainly be used in the passive voice if we mention the actor, as in “by the government”. It could theoretically be used in the passive even if we didn’t mention the actor, but it’s better to simply use it as an intransitive verb in the active voice — this is probably what your friend was referring to. Also, even when we mention the actor, my preference would be to use a more obviously transitive verb, such as “reduce”, instead of “increase”: “The burden on the public was reduced by the government.”
Between “is” and “was”, it depends on whether we’re talking about the present or the past. If we’re talking about the present, then (as always with action verbs) we need to choose between “is” and “is being”. To me, it’s a lot easier to imagine a scenario where “is being reduced” works than one where “is reduced” works — this is another reason the sentence sound a bit unusual. The simple present indicates something that happens from time to time, whereas the present continuous indicates something that is taking place now — not necessary this second, but these days. If it’s about the past, there are several aspects to choose from, but in most cases the simple past will do.