Nope, I've had moss balls in heated tanks for about six months now and they're fine.
Sorry...this is bull crap. Think about it: What healthy Betta is going to allow anything to crawl all over it as it reclines on the substrate let alone lie still as it's killed? And how exactly did the Mystery Snail kill it? I'll bet that Betta was already dead; especially since the LFS person interpreted it as "sleeping on the ground." Let me tell you, a healthy Betta would have been outta there the minute the snail touched it.I enquired about a Mystery Snail the other day and the lady there (who is great, she's quite knowledgeable) told me that if you have a betta that likes to sleep on the ground/gravel, give the mystery snails a miss due to their size.
One of her staff took one home and put it in the tank with her betta - during the night when her betta was sleeping on the ground, the snail was doing it thing....crawled up over the top of the fish and killed it.
I haven't heard of this happening before, but I suppose there's a first time for everything.
So if your betta sleeps on the ground, and your snail is quite large just be careful!!!