Il faut être très prudent lorsqu'on choisit des amis.

English Translation

You cannot be too careful in choosing friends.

Confused by this sentence - shouldn’t it mean ‘one must be careful’ instead of ‘one cannot be careful’? I don’t see where the negative is in this sentence

I agree. Maybe the translator confused “You cannot be too careful” with “You can never be too careful”, which would better match the french sentence.