What is the relation of most Fem Dommes and her slaves like? are the slaves considered as mere owned properties all the time (no longer humans) or there are other times when they can just be friends? please those experienced in this answer this question.

I think most femdoms really see thee slaves as mere property.
I never met a femdom who ever would accept a slave on the same level as she is.
Of course you can have a certain kind of friendship but a woman who ever has met you as a slave will always regard you as an inferior being.
I could say: One time slave - ever slave.
Women make a very clear line in this regard.