I would not like it. After all your "enemy" gets to spend far more waking hours with your spouse at work then you do at home. Since you clearly stated that you (KNOW) the ex wants to get back with your spouse then it's not a matter of being suspicious or trust (on the part of the ex) in my opinion. You already know (they) have a plan in place.
It's now up to you to create (your) plan. First of all you have to look at the (big) picture and ask some important questions.
1. Does your spouse love his or her job?
2. Is it a great company with lots of good benefits (high salary and growth potential)?
3. Is this a large or small company? Will they be working in the same building/department?
4. What led to their breakup? Who dumped whom?
5. Does it bother your spouse that they have to work at the same company?
6. Are the exes "friends" now?
7. Do you (REALLY) trust your spouse to "forsake all others"?
If the answer to number 7 is "yes" then you have nothing to worry about. If the answer is (no) then you have a lot to worry about because even if their ex were not working with him or her your spouse would probably cheat with someone else!
In the "Godfather" movie Marlon Brando advised "keep your friends close but keep your enemies (closer)." In this case you might choose to befriend the ex and invite them to parties and gatherings at your house. Some people go as far as trying to play matchmaker by seeking to hook them up with singles they know of.
The absolute WORSE thing you can do is blame or take it out on your spouse. It's not their fault that the company hired one of their exes! Therefore keep your plans to yourself and don't ever accuse him or her unless you don't trust your (spouse).