This would be a fantastic time for the US to drop its bs rhetoric, work with Iran, and save lives both there and in the rest of the world.
Sure that would be great but if USA offers aid, it could be viewed as an insult as if they are unable to handle things themselves and with that - pride comes into play (often does with country politics). If Iran asks for help, they would probably have the USA low down on that asking list and again, pride also comes into play.
But let's put all that aside and ask the question: what could the USA do that Iran could not do currently in this situation?
Promise not to punish anyone for breaking sanctions if they sell medical supplies to Iran.
Edit: Seems like the US will only consider that if they can use it to attack the Iranian government: https://www.nytimes.com/2020/01/30/us/politics/us-iran-sanct... ("opened access to medicine for cancer and organ transplant patients", I guess other medicine is still subject to sanctions)
Hell in the Ukraine they were throwing rocks at busses repatriating their own countrymen who had caught corona virus