Your friend needs to start by getting the title deeds (with plans) for her own property & the neighbour's. They might clarify whether what the neighbour says is correct, but often the plans are very small scale & it may be difficult to be certain where the boundary is. She can get the deeds from the Land Registry online but be careful to go to the Gov. site & not one of the many commercial ones which charge far too much.
If the deeds show the land as belonging to the neighbour your friend might have a case for taking ownership under the adverse possession rules. These normally apply only if the land has been used without the permission of the owner for longer than (I think) 20 years, & is fenced off from neighbouring land. She needs to find out the exact rules (Land Registry helpline may be able to advise), decide whether she thinks she complies with them & make a claim to the Land Registry if she does. She may find she needs a solicitor to deal with this. If the neighbour objects, there is no guarantee of success.