Were you witness to the incident?
Does she allege that you were there at the time of the assaults?
You can always make a statement to the effect that you didn't see anything. Whether the defence will use it depends.
I would generally want to call a witness if they should have seen something on the evidence of the Crown and did not. With sexual assaults obviously that is harder to do. Usually they take place in private.
If you can say anything that undermines her account then that will probably be useful to the defence in particular if you are no longer together.
Can I clarify anything for you?
The prosecution won't if you can't support their case.
The defence might.