According to Jim Recap: Mom's Boyfriend
Cheryl, Dana and Andy all have knots in their stomachs. It turns out that their mom is coming to visit. To make matters worse, she's bringing her boyfriend, and if her alone wasn't bad enough, the two of them together causes all sorts of pain every time it's mentioned. When Jim calls to say he is 45 minutes away, they all go into pain together.
Just a couple of minutes later, Jim opens the door (apparently he thinks it was rather funny) with Maggie (Kathleen Noone) and Frank (John Getz) in tow.
As it turns out, they have even more news. Not only is Maggie bringing along her new boyfriend, but she is loaning him money to buy a boat, and they are engaged to be married. When Cheryl hears this, she goes off the deep end, and a plan is made to run a background check on the guy - after all, she had Dana run one on Jim before they were married. The only problem is how to get his wallet.
While they are in the living room with the kids, Jim and Cheryl head upstars to rifle through their luggage. Unfortunately, the time comes for Frank's heart medicine and Maggie goes upstairs to get it. She catches Jim and Cheryl in the act, which doesn't go over well. In order to smooth things over, Jim and Andy offer to take Frank to the Oval Office (a strip club) for a sort of bachelor party.
This goes well enough, until they get him a lap dance, at which point he has a heart attack and dies.
It turns out that Maggie will be even less happy if she finds out that the background check turned up that Frank was actually a con man, so Jim suggests that they don't tell her. Cheryl wants to do so anyway, but she doesn't have the courage to do so, and makes Jim start the process - when she sees how badly her mom feels, she interrupts and suggests that Frank was a good guy after all. Women are so strange sometimes.
At the end, Jim tells Cheryl how he ran a check on her and found out her middle name, which is Mabel. Still no idea of their last name.
