I've seen guys mix GH with everything from sterile water, bact. water to tap water...
Is your friend getting his GH from a trusted source? Even if it was faked, most likely it would be HCG which would still mix...
As far as the experiation date, I have had friends use GH that has already expired over a year from use. Although I don't know of its potency, it still mixed in the solution.
Only thing I could think of was something effecting the GH.(i.e weather, contamination ect.) But then again most GH is sealed in kits......