OP, if you are truly a good friend, make sure you are there for the kids and put aside important mementos of their mom. If dad moves someone in very soon, odds are very good he/she will get rid of a lot of their memories of their mom. If these kids are young they will need someone who loved their mom to be there to share in her memory. Once he starts sleeping around or brings new women into the home, dad will cease to be that person. There is also a good chance he will want to create a new life that does not involve his old family, so the kids might quickly find themselves people who do not matter in the new life with his new family. It really sucks but it happens more often than it should.
They will possibly lose their father too
