apart from the legal issues, I cannot recall anywhere in the (now expired) REX EBA, which gives any scope for ICUS, un less it is done by C&T captains. Any line captain who agrees to supervise a lesser experienced pilot just to "build hours for an ATPL" needs his head read.
Not only are you helping the company out of a preventable predicament, you are placing your career and your licence in jeopardy should, heaven forbid, something go wrong.
Nothing against the cadets or not experienced enough drivers, but one must ALWAYS think about ones own self preservation at all times!