Your friend feels shy and often faces problem with group tasks/he is unable to give his best and gets left often with your partner discuss on

*the reason for his discomfort could be.
*what suggestion could him. overcome this discomfort.

The reason for his discomfort could be that he thinks his ideas are not good enough. He does not have confidence in himself which might be a result of how he grew or a bad experience he's had in the past.
He can overcome this discomfort by trying to change, bit by bit. He also needs a friend to boost his confidence so that he may share his ideas even though he might think that they'll be rejected. He will realize that rejections are not that bad and that some of his ideas are worth sharing.