Just tell her it is a semi formal event requiring heels and a dress. Or you could go shopping together and suggest she wear a certain dress etc.
We barely talk and only if I see her when my husband visits her or something other than that we never talk so shopping won't happen anytime soon lol
Put it on the invitations
Hmm did you send a baby shower invite to her already? Cuz you could include in the invite that dress and heels are required
She sounds like my mother in law. As much as I love her and I can say or tell her anything I can't say that lol. My two bosses have organised a baby shower for me at a restaurant owned by one of our clients. I am just going to tell her it's a restaurant and everyone is to wear elegant but casual clothing. Other than that we really can't say much, I would feel very uncomfortable if someone approached me about it. I guess these are the challenges of life we have to face and deal with :)
I should have put it on the invitations that was a good idea but too late I already sent it out lol