You can buy inexpensive semi-weighted or unweighted keyboard controllers for $250 or less, but if you want to get a weighted keyboard suitable for learning piano on then you'll probably need to spend at least $450. So the type of keyboard he's most interested in playing-- that is, organ/synth versus piano-- will be a factor.
Another factor will be how many different controls-- dials, buttons, sliders, pads, etc.-- are available for modifying the settings in the virtual instruments.
The number of keys is another factor, but you can get an 88-key controller for under $250, so it isn't necessarily as important as the other two factors, especially since a given line of controller will usually be available in different numbers of keys-- 25, 49, 61, and 88.
Once she finds out a little bit more about what he would be most interested in, let us know and we can make some informed suggestions.