Just relax, that's the first thing to do.
What size tank are they in now?
Is it heated and filtered?
What kind of decorations are in there?
Is it just the one molly, and what does the molly look like?
Has the fighting started yet or not?
Every betta is different, some will be relatively peaceful, others will be incredibly aggressive, and then there's everything in between. You could end up having a peaceful friendship between your fishies. Things that would help it be peaceful is if the molly isn't too flashy or colourful, if the tank is large enough so the betta doesn't get aggressive from lack of space, or if there's plenty of decorations and plants to give the molly places to hide/get out of your betta's way.
If it doesn't work out, there's a couple of things you can do. Provided the tank is big enough, you can use a tank divider to separate the two. If you can accomodate it though I'd probably recommend picking up a small tank (5 gallon/20 litre) minimum and housing your betta in there (after cycling of course).
It's important to know about fish yourself, before buying. In the end, the LFS
is there to make a profit and survive as a business. Some are more honest than others. In the future, before buying ANY fish make sure to research it and its compatibility yourself. In the end it can save you time and money with unsuitable fish.