Sorry, but I doubt that you'll find anything cheaper.
According to the Leipzig Halle Airport website, there are no direct flights to anywhere in the UK
http://www.leipzig-halle-airport.de/en/index/r eisende_besucher/flugziele.html#fz_internation al
So your friend will either have to take connecting flights (if so, �114 is exceptionally cheap) or travel to get to an airport where there are cheap flights to London.
The nearest airport which is served by Air Berlin (which usually offers the cheapest flights between Germany and the UK) is Dresden. The rail journey from Leipzig to Dresden (which is closer than Paderborn) takes about 1 hour 10 minutes on the (direct) Inter-City service or 1 hour 40 minutes on the Regional service. The return fare is �56 (about �40) on the Inter-City trains or �40 (about �30) on the Regional services.
However, for some sample dates I've just tried, I couldn't find any fares which made the journey worthwhile. (I've just researched another question about travel between the UK and Germany. On the Stansted-Munich route I could find fares of �1 each way, plus taxes & charges. However, I can't find fares under �30 each way between Dresden and Stansted. By the time you've added in taxes and charges, there's no significant saving on the fare you've quoted). It might still be worth checking your dates to see if there are bargains available when you require them:
http://www.airberlin.com/site/abflugplan.php?L ANG=eng
Chris