HVAC/Spray Foam - Consultant (greater EBR area)

TL;DR My buddy got his attic spray foamed and removed the old insulation. It's a two story story house and when his down stairs HVAC cuts on, it sucks in warm air thru the cavities of the upstairs floor/downstairs ceiling.

Does anyone have a reputable contact who can advise him on what he can do to not get hot air sucked into his down stairs every time the HVAC cuts on?

MORE INFO:
His upstairs unit crashed and he's getting quotes. He's called a few HVAC companies for quotes but they either want to simply remove and replace the upstairs unit but size it a little bigger to account for attic space. With the increased costs of everything, buddy doesn't want to shell out more than he needs to, so looking for someone who knows what they're talking about to come shoot him straight.

Thanks in advance!