First, do not assume that there is only one person involved. When the person is down and no longer a threat, you should force yourself to look around and see if there are any other threats. About half of crimes involve more than one person. You should not assume that the one that is down is the only one you need worry about.
Second, even if the police officer is friendly, it is probably a good idea not to give a statement to the police. Just ask Harold Fish, who is still in jail after multiple years and the legislature passing three different laws aimed a getting him a new trial. The first investigator reported back that it was a clear case of self defense, but the prosecutor was having none of it, and went to great lengths to put Harold, a retired school teacher with no criminal record, in jail. Janet Napolitano, Obama’s pick for Homeland Security, went to considerable lengths (vetoed two bills) to keep him there. If he had not given a statement to police, he might not be there.
The legislature changed the law to prevent this kind of abuse, but what is a night in jail compared to several years because a prosecutor does not like people defending themselves?
Always - here is my name officer. I will be happy to answer any questions when I have a lawyer present.
And make sure you get a recording and transcript of any statements.