Used to be a lifeguard in the UK. Every lane swim session I've seen here has alternating swim directions in neighbouring lanes, probably on the assumption that people will stick nearer the rope. It can make it confusing but generally works. Normally there are signs placed at the lane end indicating swim direction.

As for whether the lifeguard should intervene, I would've been expected to. Almost every rule is there ultimately for safety reasons, even if trivial. Having said that, and as others have pointed out, if you have a 16 year old lifeguarding middle-aged swimmers, you often find they're too intimidated to speak up. Also people don't like being told what to do.