According to https://www.helptobuy.gov.uk/help-to-buy-isa/faq/#998e24ea-e1f7-648f-8ce9-ff0000ad17aa :
A first time buyer is someone who does not own, and has never owned, a home anywhere in the UK or the world.
So you don't count.
The most detailed government guidance I can find does say (section 3.11) that:
the government bonus can only be put towards a first home located in the UK with
a purchase value of £450,000 or less in London and £250,000 or less in all other
parts of the UK
the government bonus can only be used towards a property that is being used for
the first time buyer to live in as their only residence and not buy-to-let. This will be
enforced by the scheme administrator
...
accounts are limited to one per person rather than one per home so those buying
together can both receive a bonus
Taken together I think that means that your girlfriend can get one of these ISAs and use it towards a jointly-purchased house. But you'd be wise to check that with the ISA provider.