Can we legally buy our friends house then sell it back to them? if so, how much time needs to pass between the?

Our friends have put their home on the market as a short sale. We are considering buying it as an investment property so they don't have to move, and then selling it back to them. What we need to know is if we can legally do that? If so, how much time needs to pass between our purchase and the time we sell it back?