I think a parent-child relationship is unique - friendship is not.
I have always guided my girls, and given them advice, and I still do, even though they are all adults now.
Growing up, we never swore in front of each other - my eldest daughter was 28 before i heard her swear, and that was under severe provocation (not by me!).
I am very close to my daughters, but I am not, and cannot be their friend, and especially not their 'mate' - our relationship is special and i would not want it lumped in with everyone else they know.