"Friend won't leave the child to cry to sleep so please don't advise this as thst won't work."
There's the first problem. A parent who is too soft to act like a parent should will cause problems both for themselves and their child. Is she is unprepared to tell the child that she is not putting up with her attention seeking and must learn to go to sleep like a good girl then there is no reasonable answer. The problem remains until the child grows out of it, or until her mum can stand not more and goers berserk. It is untrue it won't work. It is true that if the parent just keeps cracking and going in then it hasn't a chance of working. The parent is in charge (or should be, obviously not in this case) and needs to be firm.
As an aside what's she doing with a sling and not a bed anyway ? Get all sorted out in one fell swoop.
And what's this basket ? A dog basket ? A shopping basket ? I'm not saying it ought not be used but it seems to be giving the child the idea that they get whatever they want and hasn't helped.
Tell the mother to buck up and act like a parent. Decide how bedtime is going to be, insist on it being that way, and don't give in just because it's uncomfortable hearing one's child unhappy. Giving in is a grandparent's prerogative, not a parents.