Originally Posted by Bombdoll
Foam may bend or crimp, but if you use the correct sealer it wont chip. Were you planning to use EVA foam? If so seal the pieces with plasti dip; the paint won't chip easily, if at all, if you use this. Any folds or creases your Eva suffers can be remedied at the end of the day by using a heat gun. This YouTube video
outlines the benefits.
As for ensuring the armor wont bend; it should if your partner wants to sit. If they sat on a foam piece that was made to be rigid it could simply snap in half. Retaining the flexibility is what will keep this armor remotely comfortable and will allow them to move more freely and sit occasionally. Making the rear piece removable might be wise, so that they can sit for extended periods or use the loo as well. I would follow some of kamui's build videos and blogs to brainstorm the ways you would like to proceed.
Bombdoll makes really good points; you're essentially trying to fight and/or remove all the desirable aspects of the materials you're using. I can't see what you have so far since work blocks Instagram, but unless your friend wants to sit on hard plastic the flexibility of foam for the armor is going to be part of its appeal. The only other option would be as they said to make the foam armor relatively easy to remove, which is possible but could impact your design since you'd need to add additional straps or closures and figure out how to hide or incorporate them.
Also keep in mind depending on the build, sometimes you actually just can't sit in certain cosplays. From the sounds of it this shouldn't be an issue with this one, but it comes with the territory that sometimes you'll be a bit uncomfortable and have to plan your time a little more than usual for when you're going to be in or out of your costume.