I've been thinning epoxy this way for years without a problem. When this happens, just put a heat lamp on it for a couple of hours and it will complete the curing. You don't want high heat, just enough so that it's warm to the touch (probably 90 to 100 deg F.).

For future reference, after applying thinned epoxy use the heat lamp right away. This will evaporate out the solvents before it kicks off and it will cure normally.