If it's not a surprise shower you could ask her about her diet. I'm sure she'd be happy to help and pleased that you care. Or you could ask her DH (or SO) about it, too.
Everything I've read says a varied diet low in sugar and fat with lots of fruit and vegetables, legumes..pretty much what Krystal said.
Maybe for the party aspect you could have angel food cupcakes (read small servings) and strawberries and melons. That would be pretty and colorful..