Hey mindstream - is the show outside or inside and how much is she looking to spend prepping for it? Given this is her first one, I'd err on the side of low key before going too wild on a display. Get a good size table, get some table stands, including a stand that enables one to flip through prints like you're sifting through a record stack (I keep wanting to say the "thingie" because words escape me right now >.<). Black table is better for photography from what I've seen with my art friends who show photos. When they are outdoors and not using a tent, the one guy usually props some of his things at the base of the table. He doesn't show as much of his framed work outdoors since he tends to hang it (I don't think he has a tent).
Tents are money, but they allow you more flexibility and structure for showing things. Again, not sure your mom wants to go that route just yet? Baby steps! Could work to set up a metal rack stand structure, but be careful when making it. You want it to be sturdy, and if setting up on grass, make sure you are on level ground and it can take a little wind. Last thing you want is it getting knocked over and CRASH go all the framed pieces :/ I had a friend who used to show stained glass windows and I don't know where she got the metal wire frames from, but she LOVED them for hanging things. Just depends on how heavy the stuff is that you're hanging as to what route you want to go?
Get a square credit card reader, or make sure you can accept paypal payments on your smartphone. Both are awesome, and especially when dealing with pricier items, you'll make more sales. Don't rely on just cash. And figure out whether you'll accept checks, if at all (I would suggest $20 or less if you are going to allow checks, and have them put their phone number on it - easy to bounce a check.) You'll also want to make sure you know the sales tax for the area where you are selling, if applicable. Personally, I usually add the sales tax on top of the purchase price for credit card purchases (because there are processing fees), and I just build it in to the price if it's a cash purchase (because I don't want to futz with really stupid change amounts). Make sure to keep track for reporting purposes come tax time.
Once you've got your business cards designed, you can order them through VistaPrint for cheap. Easy peasy.