Feelgood ES is blah? If anything, that board might be too much for her. The Feelgood ES is one of the best female boards on the market. It uses Burton's best core plus the Vapor topsheet which makes the board super light. It is basically a female Custom X. It is super poppy and great for pipe or hardpack. This is a strong intermediate - advanced board. If your girlfriend can handle stiff, fast boards then the Feelgood ES is definitely a great board for her.
If not, then you should consider the Feelgood instead. Even better, the V-Rocker version. Burton's rocker tech is more mellow (lower profile) than others so it still holds an edge really well on groomers and steeps. It's a good introduction to rocker tech. The Feelgood is a little heavier than the ES and is also a bit softer. It still has lots of pop though. Hannah Teter, Kelly Clark, Molly Aguirre, and my fiancee's sister ride this version of the Feelgood. This is basically a female Custom just like the ES is a female Custom X.
Both the Feelgood and Feelgood ES have won numerous awards and receives great reviews.
Funny how the same people who say the Feelgood sucks will say the Custom is a good board
The NeverSummer Lotus-R is another great board. It's stiffer in the tail and softer in the nose and middle. This board is great on powder and because of the multi-flex, the board chatter is kept to a minimum at speeds. This is the female version of the male Premier F1-R.
All of these above boards are killer. The Feelgood ES might be too much for her to handle though considering her experience. That board is wicked fast just like the Custom-X. Go with the most affordable route.
Personally, I'd go for the Feelgood V-Rocker. It's just a more versatile board in my opinion. Especially if she ever gets into jumping whether it be off of natural features or in-park kickers.
By the way, I know she's currently on a Custom. But the board has changed in the past 13 years.
If you think she can handle a Feelgood ES's speed, then go for it. Just make sure she wears a helmet at least :P