I've very successfully used Plastic Surgery myself, after doing
something spasmodic to one tower on one of those startlingly expensive
Toyota distributor caps with integrated spark plug cables. Afterward,
I bolstered the repair externally with epoxy.

Note that gooping anything on from the outside of a pressurized system
is going to be a temporary fix at best. This guides the choice of what
to use (Plastic Surgery maybe, if he can get at the sides and has all
the missing pieces; glue probably not) and how to use it.

Worth trying, but keep an eye on the repairs and enough money in the
bank to getcha a new radiator...

Cheers,
--Joe