The planning department is unlikely to get involved with disputes over boundary lines. If the posts belong to you (it’s often difficult to tell, but there may be “T” marks on the plan contained in your deeds which show who owns what) the developers can’t come on to your land and remove them. If it’s a shared, or “party fence” then you will have to be consulted. But most likely the developers will put up their fence behind your posts on their land, which they’re probably entitled to do.
If probate has yet to be granted your friend should enter a “caveat” or warning at the District Probate Registry, which will mean he is alerted when an application is made. It’s possible the alteration can be challenged, and your friend should discuss this with a solicitor immediately. It may be that an agreement can be reached with the executor of the will, since proceedings in the probate court can be expensive. However the legal costs of a genuine dispute that goes to court are paid from the estate.