Frist may well have the facts on his side. Attorneys with knowledge of the case say Frist has what appears to be a valid explanation—he apparently indicated in a letter his intention to sell shares in April, long before the earnings announcement in July (a Frist aide says the senator will turn over documentation to the SEC proving this).
It doesn't matter how long before the official earnings announcement Frist asked to seel the stock, at least as long as it's measured in months and not years. The only issue is whether he was illegally acting on inside information.