Everything was perfectly legal, you have nothing to worry about except for one small thing, which really isn't an issue.
Because of the short period of time that your friend had the gun, it might APPEAR to be a straw purchase, but that would be an ATF thing, under Federal jurisdiction because it is a Federal law.
It is NOT a straw purchase in reality, because he did not buy it with the intention of selling it to you. I wouldn't worry about a thing.
You might want to do a bill of sale signed by both of you, both retaining copies for one reason: if your gun is ever stolen or for some reason falls into the hands of the police, it would be very easy for you to prove the gun was yours with a bill of sale, otherwise it might take a court order for you to get your gun back.
But, I wouldn't spend a minute worrying about the what if's, your transaction was perfectly legal.