A company that somehow has a "near monopoly" status is generally large enough that they have well defined rules with regards to "blacklisting" a departing employee from the possibility of returning. The following assumes it's a large company.
These will be defined in either the employee handbook or by HR policies which she should be able to get access to.
Generally speaking, the only times I've seen people "blacklisted" has been if one of the following situations apply:
- Terminated for Cause.
- Performance prior to leaving left a lot to be desired.
- Manager fills out the exit interview stating that employee is not eligible for rehire.
That 3rd option is generally the biggie. If she leaves things in such a way that the manager is unhappy then they will likely leave a black mark on her record; which would be reviewed by HR if she reapplies for work.
That said, I completely agree with Joe: when a woman goes on maternity leave most managers know it's a complete crapshoot as to whether she will return or not. She might very well have every intention of returning right up until she sees the baby then decides she simply couldn't leave the little bugger in day care. The thing is, anyone that has been in that situation (ie: had a baby) knows this. They leave the door open just in case the employee returns (it is the law) but they aren't counting on it.
Another thing that impacts an employees ability to return is simply demand for her job skills. If demand is pretty good in your area then the company will likely do what it can to ensure it can rehire her if the situation presents itself. The flipside is that if the market is flooded with workers with a similar skill then HR might very well skip over her just to get someone new.
The tldr; version of the above is: only the company she works for can really answer this question. Unfortunately you are unlikely to get a real answer until it comes time to try it out.