"IF" he gets pulled over by an LEO and DOES NOT have the battery on the trailer for the emergency stopping issue, HE WILL get cited, and his trailer will be redtagged on the side of the road until he goes and gets a battery, or the trailer will have to be bed hauled to the destination. Now I admit he will probably NOT have this issue.....but there is a chance.
Otherwise as noted, he does not "need" the battery to tow the trailer. The few times I have been pulled over in commercial rigs, they have checked the battery voltage to verify it is above 10.5 or 11.5v. If it was not, again, I would have the same issue as one with no battery.
Marty