It's really hard to predict. I don't think you should put pressure on yourself to decide now 'what' you should be doing. I think your friend is being kind trying to say don't worry. There's loads of time, see how it goes.
Maybe have a look at a nice evening dress from Isabelle Oliver maternity or similar instead of a bridesmaid dress, that looks like it coordinates. So you can fit bump in, or post partum bump, and breast feeding. There will be bargains to be had.
You honestly will not know how you are until the late days of pregnancy/post partum.
Ds1. Failed induction Emcs 40+10 awful pregnancy, sick start to finish, fussy baby.
Frankly, still looked & felt like roadkill until 3 months post partum. Not a chance would I be going to wedding. Photos would look like I was a ghost would turned up to bless the union.
Dd1 vbac 40+12 sick early pregnancy, got better late pregnancy. Much bigger. Yep, fine, could have got to wedding OK. No wearing of dresses though. Needed more, er, suck it all in type support. Ten pairs of spanx maybe?
Ds2. 40+14 (seeing the theme?). Felt Fab throughout pregnancy, fast birth, chilled baby. Could have organised the wedding, never mind attended.