NATO members don't have any obligation to defend or even really aid an attacked member. The article 5 is ridiculously vague. I think sacrificing the member state to avoid nuclear war is what would happen. It won't be any different than with Ukraine.
"The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all"
Every country has an obligation to defend itself when attacked. Article 5 makes an attack on any member an attack on all of them.
Short of going "fuck y'all this agreement means nothing to us" every member country has to defend against the attack as if it were on their own soil.
NATO is doing everything it can to avoid war now but i'm pretty sure everyone is preparing to react in case member is attacked