Quote Originally Posted by Eli85 View Post

Even in that situation unsolicited advice is not acceptable. It is socially acceptable to ask if the person would like your advice. If they say yes, you give it. If they say no, then you have a choice to make: continue on or find new partners and / or group. Unless the person formally agrees to it (eg., employee contract), it is not socially acceptable to start telling other people what to do, unless it is for the well-being of themselves and / or others.
I'll admit that I'm confused, because you both disagreed and agreed with me in the same sentence. It is for the benefit of everyone involved in the group activity. The person receiving the advice, as well as the others in that group. If their performance is hindering progress, then it is better to try and give them advice than to simply kick them. It is better to give them advice than to drag the whole group down.