I'd stay stick to your plan for sure, but it might not be a bad idea to bring up the school of thought that it isn't a good idea to sleep together to your ex. Be there for her emotionally and go out, have fun - but sex shouldn't be some non-negtiable part of the equation.
As long as she can follow your logic and you gently present the topic, I think it's a good move. Your coworker would feel pretty crummy if you guys end up dating and later she finds out about this. Ex drama sucks.
A similar thing happened with my last boyfriend - we went on an awesome first date and that same weekend his best friend (female) visited and they slept together. I felt pretty shitty when I found out about it.