TheGSRGuy, what you're saying is definitely true, but what I'm saying is that I don't think it's really applicable here, as our cars are capable of well over 75mph and still being stable.
what I think is happening is his tires are just really low on tread, and at speed, because the tires don't have as much grip as they should, the steering wheel is getting light and "loose," as there isn't enough friction between the front tires and the ground...yes, because of the aerodynamic lift at speed, but in this case moreso because of the (most likely) bum tires
I also just remembered that it might also be your tire pressures...so do the free stuff first: check tire pressures, check tread depth. if the pressure is off, set them to the right pressure...if the tires have very little tread left, replace the tires.
you dont need to go doing an expensive alignment if you don't know whether that's even the problem