Honestly, only you can answer this. If you feel comfortable providing a review then do so, otherwise don't but here is my take on the situation.
You have nothing to gain from this interaction that I can see. If they are already earmarked as expendable then they are probably just looking for ways to get rid of them. If you say anything that agrees with them, chances are you will be the scape goat for their dismissal.
"We had reviews that your work was not up to standards etc etc"
You can of course honestly answer dismissively
"I'm sorry, whilst I work with X I don't set their targets nor do I review their work loads. I can say however that they are a delight to work with and usually respond quickly with useful information"
An important point to remember is:
-Don't lie, this will never end well if you lie about them, you are more likely to hurt their employment as well as your own
Personally I would go with If you don't have anything nice to say then don't say anything at all.
For a few good reasons. Mainly is that they came to you, showing they already have things against this employee, anything negative you add will likely be held against you.
Those are reasons not to do it but if you still choose to give a review then i offer this:
Make sure any criticism you give is constructive criticism.
There is nothing to gain from saying "X is bad at Y"
There is plenty to gain from "X could improve on Y by doing Z" as this shows how that employee can easily change to remain constructive, this says, this guy is helpful, here is how to keep him around, rather than, this guy is useless, get rid of him.
Hope this helps