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.
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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.