Hi raining I think it's a fantastic thing you're doing for your friend but I do agree that the child should know.
We're on the waiting list for DEIVF ourselves and to get to this point have had to have compulsory counselling to be allowed to go ahead. A big part of this was round the social implications and the telling of children which was actually a really helpful part of the process.
We've always been of the mind that we would tell the child but up until we talked to the counsellor have always thought it would be 'the child's secret' for them to share with who they thought appopriate, on the proviso that you can't 'untell' information. The decision was based around not wanting the child to be followed by a label that might signal them out as different from others.
Talking it through though we've come to the conclusion that it's actually better if anyone who comes in contact with the child such as close family, friends, teachers etc. are briefed and therefore able to deal with questions/comments that might come up at an age appropriate time. That way it can become just a part of who they are and their story without any sensationalism to bring up as well adjusted a person as possible.
There's also an organisation called Ace Babes which helps Assisted conception children to meet others concieved by DE or DS to help them feel like they're not alone. www.infertilitynetworkuk.com/acebabes
This information was all given to us by the FC and is a compulsory step for DE couples as part of their licensing by the HFEA. Hopefully the clinic where you get treatment will be able to influence your friend's decision and help. You will also have to attend this step - I'm not sure whether with the couple or alone but it might be an opportunity to voice your views on the subject and for them to be mediated in a professional environment?
DE also has implications for medical support - how many times do Drs ask about family history in relation to a symptom? The child is also legally allowed to trace their donor when they reach 18. Why take away this right by creating what could become a massive desception as and when it's found out and risk the outfall I'm not sure.
I hope you come to a resolution on this. It's a tough subject and I'm sure some time and some talking will help everybody.