I just posted that vegan cake recipe (is that the one your talking about?) Any way I use buttercream frosting made from 1/4 Earth Balance and 3/4 Spectrum shortning (EB shortning has to be refrigerated to stay set and spectrum doesn't.) You may have to adjsut it according to taste becaue EB is pretty salty, but I like it like this. Also depending on how strict your friend is, you may need vegan powdered sugar. There are several organic powdered sugar brands that are ok, but some aren't quite as fine as I like for icing, and some aren't quite white. Flordia cystals brand has PS that they claim is 10x which is as fine as you can get PS, but I haven't tried them yet.
As for fondant, marshmallows are made with gelatin and are not vegan. There are a few brands of vegan marshmallows though, but I've never tried them and I don't know if they would work the same for MMF. For more info try www.vegweb.com
and keep a lookout for their adds too because there are a lot of totaly vegan stores that advertize there.
Also feel free to email me with any questions. firstname.lastname@example.org