Maybe something like, "Oh, I didn't think showers were really done after the first baby." (slight wording change from Eden that I think softens it) Or, "Are they done after the first baby? I didn't think so." And see how Friend responds--it's becoming more common, I think, for people to have them if the baby is a different gender, one parent is different, etc., and some of those reasons I personally find better than others, so that would influence the rest of the conversation for me. Though I would still end with, "Oh, it sounds fun, but I don't think I'll be able to help this time."
Not sure if Friend herself asked you to host, or some third party. If it's a third party, you could be a bit more forceful in how you turned it down. Something like, "Oh, I didn't think they were done after the first baby, I wonder if having a second one would rub some potential guests the wrong way?" Because to me the problem is, if people get invited who don't think it's a good idea, it could reflect badly on Friend and/or the hosts, and they should at least be aware of that possible pitfall.