In some industries headhunters operate all the time and its not usually to get several calls per year from agents. It is tempting to feel flattered by such calls/interviews and even offers but I suggest you try and divorce yourself from this and look dispassionately at what is actually on offer and what is at risk. To a large extent, one knows an awful lot about one's existing job/organisation but one only knows what one is told about the 'offer'. That's why its important to do some research independently or ask to talk to others in the organisation because you otherwise only know what the organisation chooses to tell you (upsides, but few/none of the downsides).
You don't say whether this offer involves working directly for this friend, (is this person a Director, for example) or whether he is merely trying to recruit you into his organisation. If the working relationship would be close consider the impact on the friendship; you would end up working for this person and at one level the equal relationship you currently have will change forever if you then work for the person (I'm making an assumption here - no-one recruits another to be their boss).