talk to her in detail about what she wants, her hopes, fears , expectations... and try not to project your own experiences on to her! you are there to support her, to keep her and DH going ,to tell them the mooing noises and wanting to go home are normal !!!
if she likes being touched, then back or leg rubbing can be soothing.. offering sips of water, bendy straws are great, energy tablets too!
and just do what she needs you to do !
she might want nothing more than for you to be in the room, or for you to be really involved and doing stuff
what she needs on the day might be different to what you discussed, so play it by ear and just follow her cues
she might be adamant she does not want pain releif , but then decides she does and you can help her feel supported and empowered when her plans change...
it means her Dh can go for a break or for a wee without worrying about leaving her alone too which is great
don;t forget to pack a bag for yourself, especially for a first labour, clean clotehs and underwear, wet wipes, a comb, sweets and snacks , change and a mobile phoen