I very much doubt that a court is going to take an 8 month old baby away from his mother to give him to another woman to care for whilst the father is at work. It's an absurd position for the father to take. If he was going to be there taking care of the baby, fine, but if not, then it's not feasible.
I presume the father is named on the birth certificate, and if so, then he has the exact same parental responsibility as the mother, so technically either one of them can have the child live with them for any pattern that they can agree on. If they cannot agree, then their first port of call is to try mediation, and if that doesn't work one of them will need to apply to court for a child arrangements order.
I cannot see a court thinking it is reasonable to do 7 days on/off for an 8 month old - at that age they need a secure attachment to one person more than they need regular contact with the absent parent. I would go so far as to say the Fri to Mon every week is unreasonable too. Moving in with someone after three weeks shows a distinct lack of maturity and that mother has a 15 month old baby of her own...I just cannot see CAFCASS or a judge thinking that the set up is reasonable.
I'd suggest your daughter stops contact unless it takes place at his parents house as it has done till now - and if he insists on involving his girlfriend, then contact should be stopped entirely as a safeguarding issue. At that stage it will be an issue for the court to resolve. The court can get background checks on everyone from police and social services and make sure the girlfriend is safe. In addition, it will give a sufficient period of time to see if the relationship will actually last as the court proceedings will likely take 6-12 months, sometimes longer.