There is no requirement for a disinterested party like JAG or the IG to be present during a counseling session
There is no regulation that states JAG or the IG can or cannot be presence during a counseling session. I personally see no reason for them to be involved during a counseling session.
If the Soldier has concerns about the counseling session they can certainly ask the chain of command for a disinterested party such as JAG/IG to be present but this is a highly unusual request. The more reasonable request would be to ask that someone else be present during the session. It is up to the individual conducting the counseling to grant that request.
If you feel uncomfortable with the person counseling you I would suggest you consider bringing it to the attention to a member of the chain of command. If you disagree with the contents of the counseling you can simply disagree with the counseling and provide your comments in writing, just make sure they are factual and professional.
There is nothing that prevents you from taking a copy of your counseling statement to the IG/JAG for review if you feel something is in appropriate. If you absence yourself from your place of duty then you need to ensure your chain of command knows where you are.