The trust is technically the fourth partner. And, the trust is controlled by the trustee of that trust. So, the beneficiaries are not the partner. Only the settlor (or creator of the trust) can change the beneficiaries, if at all. Depending on what the articles of incorporation/bylaws state, you may or may not be able to remove the fourth partner - the trust.
Without more facts, I cannot know if the trust could be dissolved, but it is highly unlikely that your friend would be able to dissolve someone else's trust.
He/she should speak with a licensed attorney in the state in which he/she resides.