First, let's get this out of the way: The GM can make any ruling he wants, and a player can either accept the ruling, fight the ruling, or depart the campaign. This answer assumes you don't want to accept the ruling nor depart the campaign.
Second, as written, literally nothing spoofs the medic's extraordinary ability medic's training. It's not a divination effect so typical counters against those effects don't work against the ability. It even works fine even against creatures shielded by antimagic fields. In the same way that the spell true seeing doesn't penetrate a simple fake mustache, making a special ability an extraordinary ability is one of the game's ways of saying Your magic's no good here!
Why the GM would make these house rules about medic's training
Let me put on my GM hat: GMing is difficult, and the way that the medic's extraordinary ability medic's training is written makes the ability monstrously difficult to manage in actual play. I've presented it again below because I want to emphasize some points about it:
Once per round as a free action (even if it is not her turn) the medic can assess the health of her allies. The medic is able to determine their current hit points, any conditions (including poisons or diseases) affecting them, and the duration of these conditions. As part of using this ability, the medic may also make a Heal check to assess the current health and condition of her opponents. She rolls her Heal check once and uses it for each opponent, comparing it to a DC of 10 + the opponent in question's CR. Failing this check against an opponent prevents the medic from assessing that opponent's health for one round.
The Heal check DC to use the ability against most foes is trivial, but if the GM allows the player to roll the Heal check, not getting information hints at the creature's Challenge Rating. The check, essentially, never fails against a particular creature—it just provides different information.
And the extraordinary ability medic's training can be used automatically every round (off-turn and not as an immediate action) to learn the current hp values of—and the conditions on—pretty much everyone. And there's no stated range limit. Every creature to whom the medic has line of sight and line of effect is subject to the extraordinary ability medic's training. (This assumes the GM's ruled that every creature that isn't an ally is at least a potential opponent and an eligible target for that Heal check—a not unreasonable assumption given the lives adventurers lead. Nonetheless, your table's rules may vary.)
I don't want to criticize design choices here. Maybe the medic's author's home campaigns are awesome kick-in-the-door-style dungeon romps so this special ability wasn't considered to be particularly valuable. (That is, by the way, a totally valid playstyle that I wholeheartedly endorse.) Maybe combat efficacy was the only thing for which the the medic class was playtested. (Caution: 9 pages!) I don't know. What I do know is that this GM would be overwhelmed by the amount of information he's expected to supply the medic every round, especially in a noncombat environment. Much of that information will be either superfluous information with no bearing on the adventure or secret information that the GM hadn't really intended a medic to learn (e.g. "Why does that vagrant have 107 hp?" and "Guys, my Heal check result on the local wizard was 30, and I still didn't get a read from him—I think we should leave!"). Either way, the medic's player is, according to the description of the special ability, entitled to that information.
In other words, most GMs (including this one!) will be forced to fabricate on the spot a huge volume of information due to the medic's extraordinary ability medic's training, and a good GM will want that information to be consistent—no GM wants to retcon by saying, like, O, yeah, you figured Baron Doolbckusi's 88 hp were just a harmless quirk when you came into town so you didn't give the amount another thought or whatever because that feels terrible… even if, in the 5 min. between the medic looking around town and what the medic's actually done in town, the GM has come up with a brilliant plot that the players would devour.
So the GM totally gets your GM's refusal to have the medic's extraordinary ability medic's training function as its written: It's a bookkeeping nightmare that provides perfect, indisputable information that sets in stone the future. It can not just spoil but ruin entire plots and even settings… and it's a special ability granted at level 1!
In light of all this, I urge discussing with the GM possible alternative restrictions on the ability that still limit it yet that don't make it—in your opinion—useless trash. A good place to start is limiting the ability to a range of 30 ft.