61.109
maybe about 10 hours. at first, I was going to say, "none", since I was going to quote the part of the 61.109(a)"...authorized instructor..." which means a U.S. Certificated Instructor, but, when I looked at the reg again, I see..."40 hours of flight time that includes at least 20 from an authorized instructor and 10 hours solo." So, any solo can be used, and he must get at least 20 more with a U.S. Instructor, so some of that other dual time could be used to fill the box for a total of 40.