I assumed she meant that because she had a mortgage, she didn't own the windows - so she only owns the percentage of the flat that she paid for (so 10% or 20% or whatever downpayment she and her boyfriend could make) and the bank owns the rest.
Or she might have meant because of the leasehold she doesn't actually "own" the property fully. When you buy a house in the UK, it usually has a leasehold - so the owner of the land under the house has leased the right to the land, usually for eighty years or one hundred years or so. So for most people, this is not a problem as they sell the house before it gets to the end of the lease and needing to renew with the property owner. When a lease approaches the end, the price of the property drops because whoever buys it next will have to pay to renew the lease. If you own the freehold, it means you own it outright (well, you and the bank that owns your mortgage).
(Caveat: I am pretty sure based on my parents/ friends buying property in the UK that that's correct but I've tried to explain it to my american partner many times, as leaseholds don't exist in the US, and I've got more and more confused so I might be missing something).