Is the following situation a redundancy or a case for unfair (or constructive) dismissal?

Scenario:

Director of company informs one of her direct reports (a senior manager, been in the company 12 months) that they are considering a re-organisation and that her department may be being combined with another. Her role might be redundant.
The new department will be managed by a colleague of the manager under threat of redundancy.

Director asks the manager to wait until she has made mind up… which could be a few weeks.

The manager feels uncomfortable with waiting and wants to resign and claim constructive dismissal.

What would your advice be?

(Thanks in advance for your input- your replies really help beef out the role plays!)
Liam Dolan