The easiest thing to do with Nikki and Abbey is to say, "O.K. that's life!" and leave it at that.
That is not the way dogs work in a human world though.
In a dog world the Alpha bitch is the one that is breeding and, as you know, at 10 months your undernourished, weak and desperate boxer sure wasn't an Alpha. Her daughter, with a better start in life and proper housing, food and support from her owners looks on Mum (sorry - Mom) as the competition.
To try to rectify the status quo do not let them off lead to play together at your house or at daughter's house. Get them together at a location that is neutral, nobody's territory. Do a little training together - a really great one for this is to get the dogs to sit and stay and then call the daughter first and reward her hugely, then Mom and reward her really well, but not as much as the daughter. You are trying to tell the daughter that she is above Mom in status and everyone is supporting her so she has less inclination to prove it. You are also showing Mom that everyone else understands the status and that she is safe and well in the position she is in. The natural reaction to comfort and soothe a dog that is "bullied" in this way only puts more stress on both dogs. Nikki is trying to show that she accepts that her daughter is higher than her but in an way that is encouraging the attacks. As she gets support from you the daughter is forced to press her point further and further. If you can back off and the daughter's owner enforce a little more control this will take a huge amount of pressure off both dogs and greatly reduce Mom's fear of an attack as she will understand, as will daughter, that there is no need for one.

Other things you can do - this is hard for the owner of the underdog, be warned, and often very hard for the owner of the higher ranking dog too but it is the natural way of things and will make both dogs feel a lot more settled and relaxed - get the owner of the daughter to totally ignore your dog when you first meet up but you, leaving Mom for a few seconds either with someone else or tied to a tree a few feet away, make a fuss of daughter. Get your dog to lie down while on the lead and let the daughter off - if she will play fetch all the better, get her owner to play a couple of throws with her. If not then a few excersises in heeling and short recall, on a line ( or flexi lead - this is what these things were made for) would be good at first. After the short play/training session (reinforces who is in control of daughter - don't be shy of correcting any disobeyed command that daughter knows!) give the daughter the signal to go play ( this may need teaching at home first but most people have a signal that they use without even knowing it. "That'll do." is the classic, but I say, "Off you go." "O.K." is fine...whatever as long as it is the signal that the dog is free to do it's own thing.)
When daughter approaches Mom you make a fuss of daughter and give her a ball throw/treat or whatever she likes and then let your dog off the lead, ignoring whatever she does. You stand up, still making a fuss of daughter and trying to keep her attention on toy/food/fetch game. As soon as daughter leaves Mom put them both on their leads, both humans fuss daughter and you go your seperate ways to your own homes.
MAJOR fuss and play for both, but especially Mom when you get home.
Do this two or three times a day if you can manage it - but make sure that both of you humans can get both of the dogs to pay attention to you when you want them to!
Take it step by step - as your bitch relaxes and the daughter becomes less aggressive you can stop being so over the top in your approach.

BE WARNED THOUGH - some bitches will never give it up when they find a punch bag who will take it and it is up to you to decide if the daughter is making a real show for show's sake or if damage is being, or likely to be done, to your bitch. This again is natural, it is simply the casting out of an Alpha that is not up to scratch.

The hardest and the most important thing is to stay relaxed and calm. If things go wrong then your friend has her dog on a flexi lead or line and can haul in if needed, but don't be put off by a bit of growling. If your bitch grovels to you looking for support, don't look at her, stay calm and let her cope with it. When she realises that she can cope with it life will open up for her and she will be a much happier being.