This is really up to the courts to decide.
I don't know how family courts operate in the UK, but it's hard to imagine that introducing a minor friend into the household makes the household more stable. I'm not really seeing how that would be a plus for a judge deciding custody to have another unrelated minor living in the home.
It's sweet of you to want to help, I'm just not seeing that this idea would be helpful to her.
I have to agree with RR. I don't think it would help much. If the system is the same over there as here in Canada, or even similar, the size of home really doesn't make much difference. They will want her to be stable financially..not wealthy or anything, but with a proven financial support. Either through social assistance, or through work. Stable also means clean of drug use or alcohol abuse. If there is a history of either of those and none with the father, it will make it harder for her. They will want her to provide a stable and clean home, but how big is not that important. A 2 bedroom is sufficient. Taking parenting classes is also always helpful.
These are just some of the basics that courts typically look for. There are always so many variables, but for the most part, the parent with the most stable home environment and lifestyle is going to wind up with custody. Good luck to your friend. I hope all works out for her and the child.