Hey,
Following on from what Castorr said you'll need to donate an additional sum which can be from £11 to £20 as donator level 2 is £10 to £19 and donator level 4 is £30+ so if you could get your friend to tell you how much he donated for you then you'll know how much more you need for £30. However people tend to donate £10 at level 2 so I presume you'll have to donate an additional £20. You can check the topic Desiner provided for the official details. Good luck!
- Tiger, Helper