Wow. What a mess. My sympathies to your friend.
What I would suggest is that the father make a Will that makes an impartial third party (with no particular ties to either sister) the executor of the Will. It could be the family solicitor for example, or a trusted family friend. That could go a long way towards making sure that one sibling does not take advantage of the other, as nothing happens without the executors say so.
What are the properties...the family home, commercial properties, holiday homes, rental properties? I think that your friend is being unreasonable wanting to hang onto all 3 of them, unless she has a very good reason to do so. Unless there is a specific reason to want to hang onto them, such as she runs a business out of one of them, or she wants to save a family or a holiday home for future generations of the family, why keep all 3? Does she plan on renting some of them out? Does she want to wait a few years and wait for property prices to go back up before selling them?
If her sister inherits a 50% share in 3 properties, I do not think that she is being unreasonable in wanting to see a financial gain from the estate in some shape or form. Why should she be forced to be a landlord, or wait years for her share of their inheritance if she doesn't want to?