I'm wondering how your son's friend is going to cope if she has no job, no parental support and only a hostel to call home. The priority seems to me to be that she gets advice as to what support, in terms of income and housing, might be available to her. Gingerbread is an organisation for single parents - I wonder if they could help.
I believe that if an unmarried man's name is on a child's birth certificate then he will be able to apply for contact, but he will also be liable to pay maintenance for the child. The CSA can set maintenance payments and if the father disputes parentage I think the Court can order that a DNA test be taken. You say that your son doesn't want to be a father but if he is in fact the child's father then, whatever his wishes, he will be expected to contribute financially to the child's upkeep.
Why does your son want you to speak to his friend? What does he expect you to achieve? I tend to agree with Mishap that, although you will naturally want to support him as much as you can, he is 23 years old and, in the main, this is something that he needs to deal with himself.
It seems the relationship between your son and his friend is not one which was intended to be long term but now that a child is involved there is a link between them that can never be broken even if they do not wish to be together. It is sad for both these young people to suddenly be faced with such responsibilities but, for the child's sake, I hope they are mature enough to remain friends and to try to sort it out together.
It must be very worrying and upsetting for you, but I hope that things work out.