Your friend has three choices that I know of.
1. Belfast City Airport
2. Belfast International (Aldergrove)
3. Newtonards.
That probably lists them in order of most usefulness, and order of price! First being most useful and most expensive.
1. Belfast City. 1829m of Hard runway + instrument facilities. 1.5nm East of Belfast. Good public transport. 2005 flight guide says £9.60 per 0.5 tonnes with Min charge £25.00. I've no more uptodate info. Road software says 7km and 9 minutes to drive to Crumlin Road.
2. Belfast International (Aldergrove) km's of Hard runways, + instrument facilities. 11.5nm NW of Belfast. Regular, inexpensive public transport into the city. From memory it was about 30-40 mins into the city by public bus service. My 2005 Flight guide says £8.16 per tonne (Min charge £12. 75). I've no more uptodate info. Road software says 23km and 22 minutes to drive to Crumlin Road.
3. Newtonards 750m Hard runway. 8.5nm East of Belfast. I'm not sure on public transport from the airfield, but I don't believe it's great. Aussie Andy maybe able to advise as he was there earlier this year. I've never been there. Road software says 22km and 22 minutes to drive to Crumlin Road.
I hope that's of help!
dp