Buying coop, have no landlord letter for pack

I've been living with my girlfriend for several years in the apartment which she was already renting when we met. We never added my name to the lease. We're splitting (amicably) and I've been looking to buy an apartment. I see that coop boards require a landlord letter which of course I won't be able to supply. I can show from bank statements my monthly payments to my gf for the rent. I have excellent credit and large cash savings, 401k etc, all showing how responsible I am financially. The co-op in this case is small, 24 units, well established, board deemed reasonable by the broker. Anyone know how boards might react to this situation? Is there a recommended way to approach this?