Hi Daffy, I was in the same situation as your daughter over 20yrs ago, I didn't tell my parents I was too ashamed, but I got chocs and flowers afer every beating so he loved me really didn't he...... I eventually got out, mam and dad still dont know everything but were very supportive. I want you to think this through from another angle if you can? My son who is now 25 still has demons from childhood, he can remember being about 4 years old and seeing me being pushed down the stairs and his dad telling him to laugh which he did because he was terrified, he still has nightmares about it because he should have protected me!! When he was 18 he found his dad and beat him up, as he was hitting him he said "this is for every punch you gave my mam" needless to say he was charged with GBH, sorry I have rambled on but maybe if your daughter think of the effect this will have on her children she may get the strength to do something about it.
Best wishes xx