Straight fingers are the same as bent backs. Got it. Playing bass is the same as power lifting. Got that too. Let's deepen our friend's worry shall we? You're right, though. I saw a bass player's fingers fall off last night, after he let them straighten for a few seconds.
I know plenty of long-time musicians who have serious health issues or required surgery due to the strain an instrument puts on your body.
I have friends who have played 40+ years without surgery, and some punk rock buddies who said "I don't need no stinkin' technique, this is about ROCK AND ROLL!!!" and wound up needing wrist surgery in their 20's.
It's not about scaring someone. It's about the reality that the motions of playing an instrument have an effect on your body, and poor technique can absolutely lead to health problems if not corrected. It's also monumentally easier to address technique issues when you're new, than after you've played for 10 years, develop tendon issues, and have to relearn the instrument to mitigate the damage you've already done.