First of all, ask your friend! She will be the best person to tell you what she needs from you.
She may want you to be hands on - massage her back, hold her hand and mop her brow, offer her sips of water, or fan her if she's hot. OR, she may want you to just "be there" as a presence. She may not know and might want you to go with the flow on the day.
Chat to her about her birth preferences and find out if she wants you to be her advocate during decision making. Does she have strong feelings about her birth options, or is she happy to go along with what the MWs will want?
Does she want lots of encouragement to avoid medical pain relief? Does she want help with staying active and upright? Does she need someone to insist on a "watch and wait" for slow progress, before beginning intervention? Does she need someone to insist she gets the epidural she wants?
A birth partner can protect the space for a labouring mother - make sure lights are kept low, the room is quiet, people knock before entering the room, and don't talk during a contraction.
A birth partner can also protect this space once the newborn arrives. If all is well, a birth partner can ensure that mum and babe get the quiet time they need after birth - no need to rush into washing and dressing the baby, whisking him/her away for newborn procedures. If she wants to breast feed, and you have experience, you could help her with that too.
After labour and birth, you can help her after by caring for her, getting her MORE tea, running her a bath, helping her get into clean, comfy clothes, brush her hair, get her some fresh pants and pad. Basically wait on her hand and foot. All the things that midwives usually want to do, but don't always have the time for in busy hospitals.
Hope all goes well - it is an honour you have been asked to perform 