Your Vulpix's IVs have not changed, because that is impossible. (To clarify, Bottle Caps don't exactly change IVs, they only elevate the stat to what it would be if the IVs were perfect. The result of this can be seen when breeding and testing Hidden Power like in this instance.)
Since IVs cannot be changed, Hidden Power typing cannot be changed either. Based on that logic, unless there is more to the situation you have described, there are only two possible explanations for this:
- The Pokemon you received back is not the same you sent to your friend, and does not have Hidden Power Ground for that reason.
- You are somehow mistaken, and your Vulpix never had Hidden Power Ground in the first place.
Some form of save file modification may also be to blame here, but I'm sure if you knew that was involved at all, you would have already blamed it for this. Regardless, it may be worth asking your friend about it -- along with whether or not he possibly sent you the wrong Pokemon.
I wouldn't feel to shaken up over it though -- as much as Ground is a good type for Ninetales-A, I think Fire is considerably better, at least for competitive singles. It hits common Pokemon like Scizor, Skarmory, Celesteela and Ferrothorn for much better damage (as well as hitting most other Steel-type Pokemon for super effective damage). The main incentive for using Ground would be to hit Heatran -- against other Fire-type Pokemon you should be switching out, as a non-STAB base 60 power attack will rarely have the power to secure a clean OHKO. Though this is all assuming you have the luxury of choosing Hidden Power types like on Showdown, which obviously you don't in this instance.