I think it's kind of awful that so many of the pregnant woman's friends are telling her she's going to be so uncomfortable at 8 months pregnant that she won't be able to sit through a 2 hour baby shower. I mean, really! Way to scare the poor woman.
Lots of women, myself included, felt great right up to the end. I certainly wasn't uncomfortable at any point during my pregnancy. I think it's silly to put that out there like it's going to happen. It's just as likely to NOT happen.
Hold the shower when you originally planned. The MTB is quite likely to be absolutely feeling great.