Well if she was actually being made redundant because of it rather than potentially moved to another post, then yes, you would argue that it was unfair dismissal if it was not a true redundancy situation.
It's essentially an internal restructuring isn't it, with a change in business requirements meaning that less of one post can be afforded in the budget? But instead of making the other person redundant they are planning to move them to an equivalent job elsewhere.
Does your friend have reason to believe she won't be selected for her existing job in the interview situation?
If she isn't selected and feels that budget and workload do in fact support and require 2 of them, then she should bring a grievance about it.
To take this as far as it could possibly go, it's difficult to see what else she could do other than the internal grievance procedure. It's not unfair dismissal because she wouldn't have been dismissed. Only possibility really is constructive dismissal but I really wouldn't say that moving someone to a different job on same terms and conditions would be considered a fundamental breach of contract of any kind. Or sex discrimination if the other person is a man and your friend has reason to believe in a fair selection process it should have been her not him.
I do think your friend has been a weeny bit premature putting a grievance in about it before the selection has actually taken place. Firstly because she may have no reason for complaint at all if she is selected. And if she isn't selected and wants to make a complaint about it, she should do it then, when the act has actually happened.
It's annoying and frustrating but I really can't see any legal problem with what they're proposing to do.