Yes! If you want to be seen as being reasonable, fair and objective then you would allow the Soldier time to respond.
Do you want to be fair? If the Soldier request time to response consider having the Soldier write in the session closing block something like:
I understand I have been given 24 hours to turn in my rebuttal to this counseling session. I understand my response is due no later than _________. I further understand that any submission after this time frame may be deemed untimely.
If the chain of command hears you refused to allow the Soldier time to right a well prepared rebuttal it may raise eyebrows as to why would not provide the Soldier time to write the rebuttal.
Elements of this issue are discussed in several posts