After all those hoops have been investigated and cleared:
Forget Nederlander and contact the General Manager**, whoever that might be.
Do it by letter (no email, no telephone) and said letter really ought to be signed by the interested party (in this case, Dad). Your inquiry ought to ask if the production is still seeking investment, what a share in the production would cost and if a prospectus is available for perusal.
The letter doesn't need to be long at all. It is an inquiry.
CC at least the lead producer** (whoever that is) if not all the co-producers**.
** The GM and the producing team might be on the show's website. They would definitely be in the Theatrical Index. Is that even still published?
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher