This female/female mating behavior isn't mating at all. Its a dominance display. When same sex animals exhibit the same sort of mating behavior a male would make with a female, its a dominance ritual. At least in any animal species research in their behavior has concluded. I can't see it being any different with dragons.
But i definitely agree that keeping males in the same cage, especially when you have a female (even in a separate cage) is asking for trouble. Also, you are introducing an unknown animal into your established collection, risking exposure to any pathogens and parasites that newcomer may have.
Best to house your friend's dragon in a separate cage and even in another room from your dragons. Or better yet, since I think you said you were only watching him for a few days, just leave him in his own cage in the other house, ask for a key so you can go in once or twice a day to feed and check up on him. Far less stress on the dragon that way, then being moved to a new environment, then moved back.
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PHLdyPayne