Well, that don't happen to me yet, but my advice, has i already read here, don't cum before it's done. If you can't control yourself is better you don't touch yourself.
From what I read, is very common a regret feeling after the husband cum, so to enjoy it for real and without doubts just don't cum to early, and from to early I mean, don't cum while he is fucking her. After he finish you can take your time.
You already decide what to do? Will you fuck her after he finish? Or what have you in your mind?
Fuck, you see another guy grab your wife, lick and suck her breasts and pussy and stretch her pussy! Your wife will suck, blew and kiss another cock in front of you, will make another cock disappear inside her, her pussy will feel a bigger cock than yours!
What position would you like most watch hem fuck her?
I can't imagine the excitement you are feeling right now!!!