Have now RTFT. So i come from a long line of military family (literally over 100 years) and i was in the army.
My first reaction is how the heck a 30 year old didn't realise the army is tough, and there is no guarantee of where you will be at any time. sorry, but it is beyond belief. Doesn't this person read the papers? watch the news?
That said. Nobody wants to be stuck having to pick up the slack for someone whose heart isn't in it. First thing: speak to troop commander, escalate through squadron (or company or whatever, depending on the regiment) and regimental heirarchy. At the same time contact the families officer and SSAFA.
if he's still in training there will also be other welfare and pastoral oppoertunities. Also the padre or any of the other religious leaders can offer (usually non-judgemental) help and practical resources.
If this person (with the Parkinson's) is actually documented as their next of kin there is more room for manoeuvre. So it is hugely important to make sure this is documented now. Preferably that it had been done at the time of signing up.
Do not ever ever ever go AWOL or "refuse to soldier on" don't even be 5 minutes late for anything. It is really important to continue to be diligent about uniform, training and any duties (guards, pan-bash whatever). The army is much much more sympathetic and understanding of people who still try to pull their weight than flop around moaning about unfair everything is. And unfortunately (it was in my day and still be the case) even with properly diagnosed mental illnesses (depression etc, PTSD not so much but that won't apply here anyway) everyone will look on the afflicted person as a malingerer. That is the nature of the military beast: the team is EVERYTHING.
Military training is really expensive, and contrary to popular belief it isn't easy to get in and stay in. And that has a flipside that they expect you to keep to the agreement you made when you took the Queen's shilling.