The bacteria and organisms in LR will die off promptly in fresh water or when dried out - SW bacteria can't survive in FW and vice versa.
How long was it dry and how long has it been in with the betta? Does it smell foul? If it was fairly new or if it smells, I'd recommend giving it a real thorough scrub and rinse, preferably soaking it for a while to let anything in there leach back out into the water before putting it in with the fish. Because of how porous most LR is, there are lots of little crevices to rinse out to remove all that, so the best method is probably to scrub, the let it sit in a bucket of water and keep dumping the water and refilling, til the rock doesn't have any smell anymore.
If you didn't do that and it has been in with the betta, you will probably want to do a water change or 2 or 3, to remove all the foul water from dead/decaying SW bacteria and organisms. Have you checked the water parameters since it's been in there? Bet the nitrates are through the roof from all the decaying stuff (assuming the tank was cycled).
As long as everything on it has died off, the rock itself isn't harmful. Since LR is such a general term, it's hard to tell how rough it may be - depends too much on the type.
