Any freshwater aquatic plants are safe for them as far as I know :)
I am SO SORRY that guy ripped you off. I honestly cannot believe that. If you don't mind my asking, where are you located country wise? If you are in the US, I would suggest checking craigslist, goodwill, garage sales, salvation army, places like that for cheap tanks! Ask family and friends if they know anyone who has one sitting somewhere you could buy cheaply.
Bettas are aggressive, territorial fish. I can't believe that.... He either has no idea what he was talking about and was just guessing or deliberately took your money....regardless I hope you have another option for buying fish related items, and boycott that place. How awful..
The minimum for a regular sized (not giant) betta is 2.5 - 3 gallons basically. My giants get 5 - 10 gallons each, and I have a 20 gallon sorority with all female bettas as well. Some other low maintenance, low light great plants for bettas are java moss, amazon swords, vals, corkscrew and otherwise... They like to sleep on leaves or in hidey holes :) I have compact and regular amazon swords in my tank and the bettas love them.
I hope you can work this situation out ad I would say yes you could divide the tank if you wanted. If you are able to fit a piece of glass or acrylic in there, you could glue it in with aquarium sealant (which comes off easily if you ever want to remove the divider) That way, no hormones or anything would get through! You could give the male 3 - 40 gallons and still have enough room left over for the sorority on the other side. You can also cheaply make a mesh or crafted divider that will allow water and hormones to pass through.
If I were you, I'd report this man and his "business" to the better business bureau in your area or a similar thing, and also review him on yelp, google reviews, SOMETHING warning others about his lies and INSANELY priced items. He's preying on people who haven't done research. How disgusting. I wonder how he sleeps at night. So sorry he did that.