My French Marans hens tend to be quite confident, dominant but still very friendly and they can get very attached. I have seen older hens that were the favourites of the removed/culled rooster show behaviour similar to grieving/depression for several weeks, i.e. stopping to lay only eating the...
I would separate Xander and Sammy leaving Sebastian with the flock and wait a few weeks to see how he will get along with the flockmaster and the hens.
In case he starts fights and causes stress, I would separate him as well and give Sammy a chance.
I would not want to keep Xander as he...