hello I am a solicitor with 20 yeats experience. I will try to help you with this.
Do you know when his will was made and when his dementia became bad.
In order for a will to be valid the person who makes it must have mental capacity which essentially means he is of sound mind (can think properly), sound memory (knows what assets he is disposing of) and sound understanding (knows what he is doing in making a will). If there is doubt about this you may well be able to challenge the validity of the will. If the neighbour is the executor as well as the beneficiary then he may be able to sell the house if he gets a grant of probate. There are steps that can be taken to prevent this. Eg placing a caveat on the property, you will also need to see the will and investigate the circumstances around the making of the will The safest thing to do is to instruct a solicitor on this quickly to make sure that what may be your property has not and can not been sold wrongly.