Hello, my name is Tony and I can answer this for you.
You need to complete Form DS1 and AP1 from the Land Registry and then send them to the Land Registry to process. Form DS1 will need to be signed by your friend to confirm that the charge has been paid off.
Once you lodge them, the charge will be removed.
There is a detailed guide on this process (although it is as simple as this) here (together with a link to the forms required): http://www.landregistry.gov.uk/professional/guides/practice-guide-31-discharges-of-charges#guide-mark-5