I would choose to be right. It could be tempting to choose to be kind instead, to not hurt people's feelings, but i do not believe in sugarcoating things. That just complicates everything.
I live in a country where English is not the mother tongue. Not everyone is good in it, but due to the Western colonization, most of us speak the language. Being in the field of research and constantly needing to write technical papers and deliver presentations in front of different types of audience, one should have good command in English, both written and oral. In this regard, whenever i hear a workmate speaking in wrong grammar, i always try to correct him/her. I know it will hurt their feelings somehow but if you think about it, it will actually help them in the long run. Because then, when they present in a different audience, they would not be laughed at.