I wouldn't trust any of the mainstream apps for sensitive information. On Telegram you have to create a secret chat for each contact/device for your messages to be encrypted end-to-end. Which might protect you from a possibly malicious Telegram in case the implementation is honest.

If the person is involved in the fighting good OPSEC practices are a must like not sharing sensitive info or at least deleting it after you have read it. It's important to preserve plausible deniability.

I'm trying to help friends in Ukraine as much as possible and advice on secure communications would be a way to do it. I know messaging apps security have been discussed in HN before but I wanted to ask the community about it in the context of the conflict of Ukraine.

The end goal for me is to give the best advice for my friends but I think it can also lead to the type of discussion HN is focused on.