Hi Yeddi
When my sister told me there was something not quite right about my 18 month olds' development, I was shocked, in denial, and somewhat put out that she dared suggest it (and my sister is a very sensitive, loving person and also a Special Ed teacher).
Having said that, I would still mention something to the Mum, keeping in mind that she probably won't like hearing it/talking about it. If my sister hadn't said anything, I wouldn't have been looking into early intervention by the time DD turned 2 and she wouldn't have made the progress she's made over the years.
So I'd raise it in private in a sensitive way and be prepared to back off if she doesn't want to hear it. If the Mum isn't ready to hear it, then it may plant a thought that may come back to her a little later on. If she has already thought about it, she'll probably say that and no harm's done. If the Mum is ready to hear it and hadn't thought of it and then investigates further to see if early intervention is necessary, it will help the child and the family in the long run.

