not really for freinds, it's really only for someone in a romantic relationship, although you might also use it for your young daughter.

I'm not familiar with the practise of calling somebody 'sweet', but 'sweetheart' is fairly common. Within a romantic relationship, you could call either your boyfriend or girlfriend 'sweetheart' if you want to, or within a family, you could use it to either a little boy or little girl. It wouldn't be usual to use it for just friends, though. It is also used between strangers. A friendly shop assistant, taxi-driver or nurse might call someone 'sweetheart', especially if they are older than the person they are speaking to. It's the same as the way many American people would use the word 'honey' to a stranger, or the way that 'love' is sometimes used in England. It's simply a kind and reassuring form of address.