As a step mom, how do I help without crossing boundaries?

My step son is 15 years old and his mom's kidney is failing. She knows she needs a new kidney but has continued to drink anyway and is just getting sicker and sicker. She had been on a wait-list but she didn't know that minors could donate with parental consent so she got my step son tested and he's a match and he's gonna give her his kidney. My husband has been in support of this saying how amazing what he's doing is. He asked to speak to me and told me he DOESN'T want to give his mom his kidney. When I asked why he said he's done his research and that he doesn't want to go through that and all she does and get drunk everyday anyway and she's never been a real mom. I told him they can't force him to give his kidney away and he said his mom is saying that she will never forgive him if he doesn't. This is all very complicated. He's too worried to speak to my husband about it because my husband supported him doing it. I just don't know how to help. Realistically I think if he doesn't want to do it don't do it. But it looks bad for me, the step mom, to say let your mother die. I just don't know how to help.