I feel a sense of death, in the area (I'm a shaman/psychic). It feels slightly ancestral to me, like an ancestral spirit may have come through it at some point.

I would have her (your friend) ask the spirit to go back through the portal before closing it. Tell the spirit you are going to close the portal before you close it. If they (the spirit) refuse to leave you need to ask assistance from medium or someone more practiced with spirits, because they will have to force the spirit back through the portal. I suspect though, if it is what I think it is, if you thank the spirit for it's help and tell it that it's work is done it will leave on it's own.

Next, the person should do this process while in meditation if they are not attuned to spirit energies. Make sure the spirit has left and gone back to where they came from. Now the person needs to pin point where the portal actually is, knowing the location is key.

Once you find the location (my gut is saying near the window but double check) figure out what it's shape is. Imagine the area you are working on as 2 dimensional reality, as if the space weren't 3D but instead was a photograph. Our regular reality will have a "flatter" dimensionality feeling. Where as the portal will feel more 3D.

Then, with your will, outline the shape of the portal as if you were using a crop or highlight tool in a photo editor, software suite. Then merge that 3D section down, to the 2D photographic image. You are essentially flattening the portal into the 2D photograph.

Mind you this is kind of advanced spirit work but it is my best How-To. If your friend is unable to locate the spirit that came through and/or feel the exact location of the portal, they may need to ask for help for a more practiced spirit worker. Feel free to PM if you need further assistance.