Both my children went to a secondary school that none of their friends went to because we moved house to deliberately put ourselves into catchment for an incredible secondary school.
What the school did, a member of staff visited them at their primary to introduce themselves and tell them about the additional things they put in place for children coming from primary schools outside their usual feeder schools.
There is always a day at secondary in July where all the primary school children spend a day with their new teacher in their new school, well there is in this area. It is then obvious from jumper colours and logos how many different schools feed into the secondary.
They also organised a morning at the secondary for all the children from non typical schools so they could see they weren't alone. They played games together to build some familiar faces and potential friendships.
Once they started secondary, a couple of weeks in they have a team building day where they deliberately separate primary group friendships to give children the possibility of making new friends.
After years of having to choose friendships from either a class of 30 or a year group of 30, 60 or potentially 90 lots of children relish new people to chat to. There are also lunch time clubs, after school clubs too to establish friendships outside of the form class.
Both my sons made friends really quickly. They are now young adults and are still friends with many of the original year 7 friendship groups. Your daughter is not thinking this through. If she lies about her address the school place can be withdrawn leaving her son with limited options of where a school place actually exists. As you seem to be looking into this, contact the school in her area and ask them about children coming from non-feeder primary schools and what they do to help them make friends. It is what a year 7 Head of Year does anyway.