How can someone who was deported to Guyana SA 21 years ago return to NY for his parent funeral?

He can apply to the US Consulate in Guyana. We can't know if they will or will not grant a temporary visit.

http://georgetown.usembassy.gov/non-immigrant-visas.html

Try to apply for a visa. If denied, they can try to apply for Advance Parole on a humanitarian basis.

Unlikely to happen since a funeral generally happens within a week after the death.

It takes longer than a week to apply for visitor's visa.