i heard that the interviews are not mandatory & if my partner has never claimed benefits would they have to attend ?

i had an interview under caution 2 weeks ago & now my new partner who had just moved in (legally) has had a letter asking them to go for an interview also, my new partner has never claimed benefits and i would like to know if my partner has to go to the interview ? i moved back to the uk in 2005 with my 2 children & have been bringing them up on my own since then,my ex-partner moved back 18 months ago & has made my life hell ever since i am going through a rough custody battle as after 4 years my ex-partner wants the children back (they have never paid or helped out financially towards the upbringing of the children) my new partner was going to move in at the end of january but due to my interview we decided that we would move in together 2 weeks early my new partner stays with various family members during the week & with me at weekends all household bills are paid by me & are all direct debit which can been seen on my bank statements my ex-partner knew that my new partner was moving in and has reported me for benefit fraud out of spite (my ex-partner has been caught twice claiming benefits & working & it would look like i am a bad parent in court if the investigation was successful & give them ammunition for custody)) so i would like some advice on whether my partner should go to the interview?,will it affect my investigation if they refuse?,what they should say if they do go & do they need a solicitor with them at the interview ? with all the stress of a custody battle i don't need the stress of an investigation also