Be careful.
When anyone leaves a long term relationship, there is a feeling of insecurity, and that naturally leads the individual to look for a situation in which they felt safe, and knew where they were with the other person.
A recent ex is the perfect individual - familiar, safe, comforting, and a re-affirmation of desireability and attraction.
Trouble is - it's not real, and it doesn't last. before you know where you are, you are both scratching open the wounds that separated you in the first place.
If you still feel anything at all for this woman - or you don't want to take advantage of her vulnerable state, then accept that she is not the one for you at this time, and you are not the one for her - it will end in tears.
If however, as some other responces have suggested - you have no concicence, and will accept rebound sex when it's on offer, and you can walk away unscathed when it ends again, then by all means go for it.
I think I know which option you are going to go with.