She definitely needs a low 'A' - mine's a Shaw, but other makes are probably as good or better. It enables you to play a lot of tunes in 'D' while using the normal 'G' fingering of a 'D' whistle.

And a low 'G' gives you the same option with tunes in 'C'.

Perhaps an 'E', but the one I have is terrible, so I hardly ever use it