Hi,
I would also agree that Ghana is a great place for a gap year; politically safe, very friendly welcoming people etc. I lived there for three years and absolutely loved it.
You do not have to go the High Commission in Highgate to obtain a visa - I have always downloaded the forms and sent them off with passport, fee and pictures - never had any problems.
She must, must, must take anti-malarials and any other jabs recommended. I would recommend going to a travel clinic rather than your gp (although your gp can most likely refer you). I had malaria twice whilst there and it was horrendous. In Accra (the capital) there are some very good private 'Western' style hospitals, but elsewhere facilities can be very limited.
You can fly to Ghana from the UK with British Airways, KLM, Emirates and Afriqyah (Ghana Airways went bust a few years back). I've always flown with BA, it will cost approx £550 return. I would recommend though paying extra for a flexible ticket where you can return at any time for a £50 fee.
The other danger in Ghana is traffic accidents. Locals travel around in 'tro-tros' which are small minibuses, packed full of people. These are in varying states of decay, it is not unusual to get into one and see gearboxes/windows held together with string. There are always stories of tro-tros smashing into something due to their brakes failing or coming off the road. I would advise to always use the large coaches to travel long distances and shared-taxis for shorter distances.
For places to visit, I would recommend all of the above. Whereabouts will the orphanage be where she will be working? There is a fantastic beach resort in Kokrobite, called Big Milly's, that she will absolutely LOVE. Another great trip is to take the ferry from Akosombo to Yeji along Lake Volta. There are only a few private cabins so you must pgone ahead to book them.
She will get approx. 50 marriage proposals a day! All done in a totally non threatening way, but gets very tiring after a while. As does getting called 'oubruni' (white person) - children will literally chase you around shouting this in some villages!
There is a good new guide to Ghana which includes volunteering and has up to date prices for everyday items - you can download it for about £5 - www.g-lish.org
I hope some of that helps, feel free to ask anything else. I am going back to Ghana in November and can't wait!