"My understanding is that any redundancy of someone taking mat leave will almost always be judged unlawful and discriminatory unless the entire team/branch/dept are going too."
Not at all. It's perfectly lawful to make someone on maternity leave redundant, as long as it is a genuine redundancy situation and their maternity leave was not all or part of the reason for the decision.
There is additional protection in that if, at the time the post is redundant, there is a suitable alternative vacancy available, the woman on maternity leave must be offered it.
OP were these other jobs suitable and available at the time your colleague was made redundant? Or have they come up since? If it was since her employment ended then management are right in that she is no longer an employee and she doesn't have any right to preferential treatment.
Now her employment has ended, it's up to her when she ends her maternity leave and goes back to work, either to a new post at her old employer or somewhere else. If she applies for a job during her maternity leave she can ask the prospective employer to delay her start date to enable her to spend more time with her child, but they are not obliged to allow that. She won't be "on maternity leave" from a new employer (whether it is the same employer as she used to be with or a different one) so she won't have the right to decide to "return" after up to a year off as she would if she was on maternity leave but still employed.
The questions basically are was it a genuine, fair redundancy situation, and were these other jobs suitable and available at the time her employment ended?