In Act 2, Scene 8, Waitwell and Foible arrive in the park. Mirabell lightly admonishes Waitwell for being late, and Waitwell responds in a way that highlights the situational irony of his happy new marriage:
MIRABELL: [....] Sirrah Waitwell, why sure you think you were married for your own recreation, and not for my conveniency.
WAITWELL: Your pardon, sir. With submission, we have indeed been solacing in lawful delights; but still with an eye to business, sir. I have instructed her as well as I could. If she can take your directions as readily as my instructions, sir, your affairs are in a prosperous way.
Waitwell's response is supposed to be comical: as was common in Restoration comedies, Waitwell makes a bawdy joke about his satisfying sex life with Foible, his new bride. "Solacing in lawful delights" simply means that they have been enjoying the fact that they are legally allowed to do whatever they want sexually now that they are married. Waitwell also hints that Foible, who has essentially married into Mirabell's service by marrying Waitwell, is going to make a good servant because she is good at following orders in the bedroom.
The crass jokes serve more than to delight the audience. They also demonstrate that Waitwell and Foible's marriage is going better than might be expected given that it came about as part of Mirabell's plot. Ironically, a marriage designed to trick people is actually a passionate marriage. By contrast, the other marriage represented in this act was Fainall and Arabella's marriage. The two of them supposedly have a more "real" marriage than Waitwell and Foible, but Arabella joked that she and her husband would spark rumors if they were seen walking together. The "real" marriage seems to be utterly devoid of passion if Fainall and Arabella are so rarely even seen together.
Waitwell's joke about the intertwined nature of sex and business also has a serious note: in the social environment Congreve was trying to capture, business, pleasure, performance, and authenticity are all intermingled. This is not necessarily a bad thing, but it can be. In the case of Waitwell and Foible's marriage, the fact that business is part of it is no problem because they have a genuine relationship as well. What Congreve criticizes is marriages like Arabella and Fainall's, which clearly have no basis in love or passion to accompany everything else.
In Act 4, scene 15, it seems that Marwood has at last foiled Mirabell's plan by delivering Wishfort a letter revealing that Sir Rowland is really Waitwell in disguise. In a twist of situational irony, Foible and Waitwell are able to use Wishfort’s hatred of Mirabell to further convince her that Sir Rowland is a real person:
Oh, what luck it is, Sir Rowland, that you were present at this juncture! This was the business that brought Mr Mirabell disguised to Madam Millamant this afternoon. I thought something was contriving, when he stole by me and would have hid his face.
Wishfort's deep suspicion of Mirabell and his ethics should lead her to spot his manipulation. The entire point of Marwood's letter is to disabuse Wishfort of the belief that this sham uncle of Mirabell's is real. This whole thing is a setup, and Mirabell is behind it all. This really is manipulative behavior on Mirabell's part, and Wishfort has reason to believe the worst of him. Ironically though, her low opinion of him backfires on her. Knowing that she will believe the worst of Mirabell, Waitwell and Foible convince her instead that Mirabell wrote the letter as another manipulation tactic. Supposing that Sir Rowland actually is real, Mirabell has an incentive to convince Wishfort that he is a fake so that he won't cheat Mirabell out of an inheritance. Wishfort, who desperately wants Sir Rowland to be real because she relishes his romantic attention, is all too ready to believe this version of the story. Where Marwood and the audience expected Mirabell's entire scheme to come crashing down with the delivery of this letter, it instead flourishes.
Not only does this make for a fun and surprising plot twist, it also allows Congreve to satirize the way a petty grudge like Wishfort's can lead to reductive thinking and underestimation of a rival. Wishfort's opinion of Mirabell is a little too low, at least in terms of his cleverness and competence. He is capable of far more than writing a fake letter to get what he wants. He has created an entire fake person to carry out a Restoration version of a modern "catfishing" scheme. Wishfort makes it easier on him than it needed to be.
