Many deed plans do not have t marks. This does not matter. The boundary is the red line and the neighbours cannot encroach on her side of that boundary. From what you say, it seems that the entry is to give joint access to the back gardens. The deeds ought to give information on its ownership &/or use, but if the house is an old one they may not do so.
She should not leave this. She should see her solicitor (preferably the one who did the conveyancing when she bought) for advice. It may well be necessary for the solicitor to write to the new neighbours (or, if they have not yet bought, to the present owner and the estate agent) making the position clear.