Keep it simple when dealing with border guards at say LHR. That means not volunteering info that hasn't been asked, and being polite as due, but not really worrying about it.

While the landing card has a question about work, saying "real estate" would be sufficient. If asked more questions, "develop property to flip it" works, without saying "self-employed".

If asked for how long staying, give the right answer. If asked the purpose of the visit: "visit SO and some other friends".

As long as the person seems destined to leave on time -- ticket heading back to the US helps -- without violating the conditions of the entry -- funds to pay for the stay and not become a burden on the UK -- it's usually not a problem. At least for guys in this kind of situation. For gals in this kind of situation, it's a bit of a different beast.