Well as long as no-one is mentioned personally then I think thats fine. I think that when you write a song, or whatever, it's your own way of coping with what has happened to the lassie and her family. OK she may not be a close friend but she has changed you forever and I suppose the song is a way of expressing how you are feeling just now.Then puting yourself into that position. However I think the thing would be for you to ask your friend and see what she thinks of it and wether or not she thought that it would be acceptable for you to sing it to the woman who lost a child. Only thing being that it's such a painful thing for those who have been affected by it. It may lead to a negative response especially if the lassie doesn't know you that well she might feel an intrusion to her grief. But for me being a guy I don't think that I would but then again we think differently from women.
I hope you choose wisely
Dylan