I can understand how hurt you must feel at what your dad's partner is saying (I lost my mum on 21 Feb ). But the only thing you can do is to talk to both her and your dad about their wishes - it may be that this is what your dad wants. I think I've read in a leaflet that the ashes are returned to the next of kin which will generally mean his wife or partner, and it is up to that person what they do with the ashes.
Do talk to them both especially your dad - it will be the only way you will find out what they think.
Good luck - let me know how it goes.