odd behavier in a horsfield

I have a male and female horsfield and a male herman, in the last 3 weeks the female horsfield keeps trying to mount the herman she throws back her head and makes loud squeeking noises just like a male would, the herman just sits there and ignores her it happens about every 2 - 3 days they are all age 6 the male horsfield keeps out of it, inbetween times she often lies with her head on the hermans leg, they all eat well and live in the greenhouse with free access to the garden have you any advice please. Joan