First of all, Jenny has all of her father's flaws, but none of his redeeming qualities. For all that Charlie was a womanizing drunk, he was also a generous provider who took care of the people he loved. But Jenny is a mooch. So she actually embodies the worst parts of both Charlie and Allen, without any of the good parts.
Secondly, it was already stretching credulity that Walden would put up with Allen living in his home. That Charlie would endure Allen and Jake made sense: they were his family, and for all of his selfish, irresponsible ways, Charlie actually did have a sense of familial duty. Walden was grateful to Allen in the beginning for being a friend when he was lost without his wife. But they're well past that now, and that debt has been repaid in full. And there is no obligation at all with Jenny.
So why would Walden put up with Jenny and her lovers caterwauling and keeping him up at night? Why would he endure Allen throwing hissy fits and guilt trips because Walden won't pimp out his mother's friend? Walden was right to say no. Allen had shown nothing but creepiness towards Lynda Carter, saying his poster of her looked like the bottom of a bird cage and that he wanted to "bone" her, and acting like a very crazy fan when Walden had made him promise not to act like a crazy fan.
Now when I watch the show, I'm just rooting for Walden to throw them out. Even if he feels too guilty to toss them out onto the street, the guy's a billionaire. Why doesn't he just buy them a two-bedroom condo and toss them into that?