lol this has nothing to do with their dog beds.... its simply that the bitch is for some reason taking a dislike to the boys behaviour. You should correct the bitch before the attack begins, by this I mean you will be able to see when shes got her eyes on him and you should give a steady, leave, no, ah ah command whatever it is you use.
I couldnt say why ses doing this - maybe she thinks hes getting more than her, maybe she thinks he will receive extra treats from you - the bitch in 99% of cases will always dominate the boy but that doesnt mean you shouldnt step in when its taken too far especially when the boys getting old. Simply because he would guard the house, doesnt mean the bitch wouldnt get her own way in other circumstances, id take a bet this was the case at least. You want to have him able in furture year not to be bullied. Stopping it now will help hugely in the long run and shouldnt take much telling. Be very firm though and if she doesnt listen, remove her, its should sink in