I'm going to tell one of my best friends he's making a mistake marrying his gf
Why would you say that to your best friend?
I believe it's a small private gathering within a closed circle of friends, and I don't think that you should hold a wedding ceremony at a time like this.
It's great you're concerned for their health in a pandemic like this, but this is their day and you shouldn't ruin it. It's their relationship, not yours and if they choose to go along with this so be it, it's not your wedding. You don't have to attend if it's not safe.
True, I think I'll stay home and nuke myself.
That might be the best solution, as long as you tell your friend why you won't be attending.