My girlfriend's dad won't put her on the house deeds even though she inherited half from her mum - will this mean a bigger IHT bill?

My girlfriend's dad won't put her on the house deeds even though she inherited half from her mum - will this mean a bigger IHT bill? →

She has not put her name on the deeds yet, as her father said wait till he dies then she will be able to change the whole thing at once.