If your mom really wants some extra help, maybe she could just ask your friend point blank. Or even you could. I know this isn't the way things are supposed to be done, but none of my friends would ever be offended if I just said, "Hey, my mom is throwing my shower, but is there any way you could help her with some ideas?" or whatever the case may be. I also wouldn't be bothered if one of my friends asked me in this way.