I'll be the dissenting "yes." Your mom wants to be generous with the "special" speaker. It sounds like this speaker is not the traveling overseer, but rather someone who was invited specially from Bethel to give their "special talk."
These guys are not making a haul, not traveling first class, and probably have agreed at considerable expense to themselves if the congregation is not a rich one. (Probably even if they are a rich one.)
My parents accepted these "invitations" a few times, and always went back to Bethel hundreds of $ poorer than before they accepted. Usually they get their airfare paid for (or mostly. My parents were asked if they had airmiles, which they did, so the congregation expected them to use their miles, which they WERE going to use for vacation.) Usually they get to stay at somebody's house and sleep on an air mattress (whoopee!), and they get taken out to some crappy-assed family restaurant or go to "potluck" after the talk. But nobody really helps them with eating expenses for the travel days, car rental (if needed,) eating expenses for the rest of their stay- usually an additional day or two. This isn't the cushy job that being a C.O. can be. The guys who take these talks, some of them, are the good ones... the ones that sacrifice because they think they are doing it for God.
Your mom sounds like she has a generous spirit, and she wants to help somebody else who quite possibly has a generous spirit. Yes, they're in a rotten stinkhole of a cult, but they're people.
If you decide to not give her the $20, (which is not much all things considered,) please just tell her you're not comfortable passing your money through to a group you can't support. But don't play the little "i forgot" game. She's doing it out of love for her "brothers." That's really what it's all about.