Forum Immigration France

[Conrad24]

My girlfriend is 1/4 French, 1/4 Spanish, 1/2 non-EU; I want to take to to the UK, but if she has no EU passport then it is a big problem. We try to find all informations how she can get French citizenship and passport. Her grandmother married a Spaniard, lived in Spain for few years, had a daughter (who is a my gf's mother), moved out from Spain. We don't know if she(my future mum-in-law) has French citizenship, undoubtedly she has Spanish citizenship, but looking on forums to get Spanish passport for my girlfriend will be time consuming so we try to find how we can do it with her French passport. If her grandmother is a French citizen, maybe my gf is entitled to French passport also?
Does anyone know it?
I asked French embassy and got useless info.