Lexus will drive fine. Limp mode is rich; but its not as super rich as everyone is describing. You can drive a Lexus into the dirt and it will still run. Don't worry about it, but replace the O2 sensor soon enough.
I ran my SC300 on limp mode on purpose when it was turbocharged because it ran a little extra rich and less timing. He'll be fine and it won't damage a thing.