Without question the Piano.
The fundamental instrument, on which everything else becomes a reference point to, especially in theory, chording, harmony and arranging...should she have ''the gift'' and want to pursue this for years to come.....and it is one of the easiest instruments to learn and get a GOOD sound out of, quickly and with minimal effort.
(But I am of course biased, as I started on piano at age 4...however it facilitated the learning of other instruments, enormously.)