Well, the unfortunate thing is that unless you know what you are doing, this idea wouldn't work. I have actually had to advise someone on how it is unethical to do it. Given that the bride was ...... i told her that i would take care of the fish .... so what exactly did i do? I went and purchased them the DAY OF the wedding. I also took water from my existing tank to help with everything. What i also did was have the bride buy some small tanks (enough to house the betta) and have them given to the children who attended the party. This stopped the immature people from dumping crud into the vases. Next i had her put water lettuce in the top od the tank (good root system on it) so as to not suffocate the betta. Next i had her place her flowers around the vase.
So when the dinner was over and the kiddies were leaving, they each got thier own betta to take home with and aquarium plant (or 2). To help with this also i threw in some substrate from my tank to help out with bacteria.
Just being the nice guy that i am .... i wrote out, for the kids and parents, instructions on WHAT they need to keep it alive and HOW to clean a tank properly.
Any other way and i wouldn't help her.
Lunchbox ... "i'm a man, but i can change, if i have to, i guess"
Tankless ... for now ... until i find what i really want (and what the wife lets me get!)