The recent fare changes in the Lea Valley have altered nothing about season tickets, only Oyster PAYG fares. Fares from Cheshunt are confusing, but that's the way they've been ever since Cheshunt joined the travelcard zones two years ago. Since that time there've been two season tickets available, both with the same price but with different validities: Cheshunt to zones 1–6, and a simple zones 1–8. The former is what Greater Anglia were selling, but the latter is better value for money as the extra two zones can come in useful for travel to places such as Watford (High Street) too should that be needed.
Most likely what's happened now with the takeover by London Overground is they're ceasing retailing the travelcards with specific station origins, and transitioning to the simple 1–7/8 tickets instead, which is preferable. (Cheshunt station will be an exception to this as it's still run by GA, but that doesn't mean that they can't technically retail a 1–8 ticket, as they can, albeit not on Oyster.) Unfortunately, travelcards that exist beyond the standard zones 1-6 are a bit complicated in the world of National Rail: they're sold with a destination of zones 1–6, and an ‘also available at’ route containing the extra zones – this is what your discovered with the ticket machine.
So, to cut a long story short, your girlfriend will most likely just want a simple 1–8 travelcard on an Oyster, and could have had this since 2013 but it just hasn't been very well publicised. A useful side effect of the travelcard zones is that a 1–7 travelcard is valid to board at Theobalds Grove and go to London via Cheshunt for free, provided there's enough PAYG credit on the Oyster to allow the holder to make a hypothetical journey from TEO to CHN.