I think your friend meant Cl2 (not C12) which is the native form of chlorine (Cl will form a covalent bond with another Cl forming Cl(2)) Cl+Cl =Cl(2)...
Chlorine (Cl) is never really on its own because once it is broken down from Sodium Chloride (NaCl), the solo Cl ion seeks out another Cl unless it bonds with another metal (like K or Na)..
In most cases the Cl ion will go for the covalent bond (where the bond is due to sharing a pair of electrons) before it will go for the ionic bond (based on electrical charge)
Cl ionic bonds..
Cl has a charge of -1 , so it will bond with elements that have a +1 charge (Lithium, Sodium (Na), Potassium (K) etc)
Cl(2) has a charge of -2, so it will bond with elements that have a +2 charge (Magnesium(Mg), Calcium (Ca)
Your friend put Sodium Thiosulfate in the water so that the Sodium would bond with the Chlorine... and it could become NaCl... that would be disastrous form of sodium for your plants....
Lol my first post-graduate degree is in Hydrogeology so I got a decent handle on chemistry