I think the reassurance re not sleeping being normal and very easy to resolve is the most important advice.
If possible help her not revise in her room. So when she goes to bed its in a revision free space. Then suggest she gets comfy before bed, however works for her (, cool bath or shower is better than warm, cosy book, listen to music, meditation app if she likes) ideally have no screens or tech in the bedroom. Then when she feels drowsy switch off the light and go to sleep. But if she's feeling unable to sleep after 10-20 mins its fine to get up, read for a bit, go for a wee, get comfy and try again when she's ready. Lying awake worrying will just ensure she carries on lying awake worrying...
The cycle is usually one bad nights sleep at a time of pressure, followed by worrying the next night you won't sleep due to the worry meaning the worrying about not sleeping adds to the exam worry and so on and so on...
Sleeping tablets can help break that cycle - so the other thing to do is give antihistamines at the weekend when there's no exam the next day. Ideally don't let her sleep in too late, get her up before 10 so she is still tired at her regular bedtime. But emphasise its not an every night solution, its just so the next night she has the reassurance she can sleep well and will.
Also have a slow release energy snack before the exam (banana, oatcakes, bagel with PNB) for the reassurance she won't get in there and nod off! That and the adrenalin will see her through.
All three of mine did this/do this. So do I when stressed so I know if you can just sort out the first bad night she will be OK.