Simple answer – be antisocial and don’t make Friends – then you wont have to delete them when you fall out !!
or
Yes, you cannot delete your Friends from within MSFS, but, far more powerfully —
You could do what some have correctly suggested, install the X-Box app, (if you have not already), and you can Manage your friend list there …
AndrewJ5664 selected — one click on “Remove Friend” and he would be history !!
Note: They are NOT MSFS Friends, they are X-Box Friends …