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.

They are both correct, you could say: “the burden on the public is/was decreased by the government.” (passive use of decreased) or "the government has decreased / is decreasing the burden on the public." (active use of decrease or decreased) As far as using "is" or "was" : "is" is a present tense form of "to be," so it would be used if you are making a statement about the present. "was" is a past tense form of "to be," so it would be used if you are referring to an event in the past. A few examples: "Studies show that when these conditions are met, the burden on the public is decreased by the government." or "When the policy x was enacted, the burden on the public was decreased by the government."

"Is" is the present tense form of the passive verb. "Was" is the past tense. They're not interchangeable. I would only use "is" in certain cases, like in reporting events or habitual actions. "Was" is for, again, something that occurred once in the past. FOR EXAMPLE: "IS" 1. Reporting an event. "Spending is decreased by the government on arts by 4%. People are mad that this occurred." This is talking about something that happened in the past, but you use present because you're creating a timeline. This is possibly more active than the past form. 2. "Spending is decreased every year by 4% by the government on arts." This is habitual. You can tell because it says "every year." You could also use past tense with this, but it would have a different meaning, and using it with present is a special case I wanted to note. FOR EXAMPLE: "WAS" 1. "Spending was decreased by the government on arts by 4%. People were mad that this occurred." This is more passive in my opinion as a native speaker. It only happened once, and this form is very "reporter-like" in that the verb distances itself from the subject. I hope that helped you. If you have any questions, feel free to PM me.