I myself was one of those Mum's who wouldnt leave my son! I was a single mother and the father never wanted him! I was extremely overprotective, and the phase lasted until he was about 7 months old.
I was told the same thing, about leaving him with people and I refused aswell.I am still breastfeeding him (and have done from birth) and I was also too scared that something would happen to him in my absence. Nothing frightened me more than something happening that I was not in control of!
I dont have an issue leaving him for small amounts of time these days, but in general, my son goes where I go! We are a team and I dont think that is a bad thing.
Yes, he loves Mummy more than anyone and if he is sick or teething, Mummy's the only one he wants. But he is a very happy baby and goes to everyone with a big smile!
I think leave her alone and she will some around in her own good time! I can tell you from experience, that the more someone harrasses you to "leave them alone, and let go a bit" the more aggravated it makes you...and a whole lot less likely to leave the bub with that person in question!!