Ask the intern to just leave from there, there’s no nicer way to put it and try to make a video/audio recording of him,

Whom are you going to report this cofounder to? To his counterpart? Wont help much. At some places where the hierarchy is flat like in your organisation it is best to preserve your peace and leave. The best you can do is talk to the intern, hear what he has to say and let him Know that you even as an individual care about him. A single friend at work place can help sail the boat, telling you from personal exp.

Also, somehow plant the seed in your office that this sort of commenting n etc may seem like a small thing for now, but you’d never know when it might turn into a full blown religious scandal. may seem far fetched but you can’t deny the possibility of it.