D.G.
Don't take it out on the kids until you find out what REALLY happened. Girls can be bratty at this age and say inappropriate things.
Go visit the mom and strike up a friendly conversation. Butter her up with lots of compliments about her daughter: she plays so nicely, my daughter likes her, blah, blah. Then say something like: You know she said something interesting the other day, and I was wondering if you could shed some light on it... she said, '... liar' comment. I wonder if my daughter said something to Megan to upset her or there was some kind of confussion that we could clear up, yada yada. don't pin in on Megan and her mom, just suggest there was a mistake.
If mom is a b... about it, drop it and move on. Let the girls have a friendship without the pissy mom. don't write off the relationship b/c of ONE comment. If things continue... then consider dropping them. Hey, neither of you are moving anytime soon!