* My question is, how safe is it to keep screenshots of Bitcoin/crypto transactions and useful information, on ones device?
How important is it to do. If it's not important, it means it's okay.
For me it's not safe unless he has two smartphones with different functions.
* How do I go about letting my friend know about such a habit of taking and keeping screenshots on his device that I have observed and warn him about it, without looking like I sense bad days coming?
When a cellphone changes hands, not because it is stolen or lost, today's smartphones generally have advanced security for initial access because they have several types of security to be able to open it, from PIN passwords, patterns to face scans. There is another concern if we often take screenshots of important information such as Bitcoin asset information, crypto and other financial transactions that are still private when the person holding our smartphone knows the password to open it.
If the party who takes our smartphone still has no negative intentions, you're lucky. If our smartphone falls into negative hands, woe to us with all the important information we have screenshotted.