[bioengine]

Swiming uses a pulling motion, researchers say that swimming is isometric movement patterns. Golf is power explosive.
I can see how Hogan believe swimming isn't good for golf which has merit in what he says.
Although your student if it was a timing and rythm issue swimming could have some effect to how the student is firing theri muscles in the golf swing.
If it's right lateral bending and they are getting hyper extension and hyper flexion it's more of a movement patterns issue. this could be due to poor lower body mechanics, not bein able to create ground forces or creating lower body stablity.
Another possiblity is your student doesn't know how to turn their body around the axis of their spine.
There could be tons of factors to your students issues.
By the sounds it's more a movement pattern issue than swimming being the cause. Your student need to learn general movement patterns in general.