First, for anyone reading this and planning a wedding, this is why so many of us recommend not rushing into naming your bridesmaids. Sure, if you're super close to your sister, that's one thing, but there's no valid reason to do it right off. Too many things can change, including major downsizing of wedding once sticker shock sets in.
To the asker, you don't say when the wedding is, but if it's still a ways off, don't make the same mistake twice. Wait until you're more sure about the friendship. If it's later this summer or whatever, just ask her. Why would she assume she's Plan B for someone you dumped?
Finally, I don't know what you mean by bridesmaid "duties", but actual duties are very limited: buy the dress you pick and show up for the wedding. Lots of bridesmaids do more, of course, but this is only if they offer and they want to. I don't know if this was part of the reason your friend dropped out, or if it was something totally different, but you do want to avoid whatever caused it to go south the way it did.