[RPG] How valid is lying to players about the rolls they are making in The Dresden Files

It's still an attack, so an opposed roll is still required. Since it's an attack evocation, your "attack roll" uses Discipline. The only difference is that you're not also using this same roll to control the energy of the evocation: you automatically get a roll of 0 to control it.

So yes, you still roll Discipline in an opposed roll against the target's defense, and if you win the contest, you get to add the rote's weapon rating (4) to the shifts you got in the opposed roll to determine the total stress inflicted.

From the writer, Fred Hicks:

biomancy is a tricky, poorly documented part of magic in the source material. But, yes, it’d incorporate transformation magic, and its healing elements likely manifest as accelerated medical care, only occasionally branching into true “regeneration” type stuff at the extremes.

With that in mind, I think that a Biomancer is more akin to a doctor than a cleric, which seems to fit the source material. Keep in mind, that this is an urban fantasy, so having an easy way to heal an extreme consequence (an arm being taken off, for instance) brings to mind the question of why is cancer still a problem, and (without going into too much detail) why are extreme wounds to characters in the source material such a problem.

That said, I'd look towards the character concept and the focus of the player in creating the character to how you might tailor things to make the concept work in your game, and make things satisfying to the player in combat.

In the Dresden Files combat, there are four basic conflict actions: Attack, Maneuver, Block, and Sprint. (ref YS199). Taking sprint out of the equation and reducing the actions to their simplest intent, these actions are meant to either protect the character (thus prolonging the conflict), damage the opposition (thus contributing towards the end of the conflict), or set up opportunities for one or the other.

An application of Biomancy that would fit within these rules is a Block. If someone is in combat, the Biomancer could focus himself on increasing their natural resistance to damage and their natural recovery. The description of the effect would reinforce the use of Biomancy i.e. instead of a kinetic shield stopping the bullet or altering it's path, the bullet impacts the target, but the biomantic energies were able to knit the damaged body before the effects of the shock could even register.

If this is not enough to satisfy the concept and you want to extrapolate the rules and/or use the FATE SRD rules, you can move beyond this. An example of mitigation of consequences is already given- and I think given the statement above about what the intent of what biomancy would be fits their example. So I'd concentrate on stress.

Looking at YS201, we see a definition for stress:

Stress is an abstract representation of the difficulties that threaten to take someone out of a conflict. In a physical fight, stress can be minor cuts, bruises, fatigue, and the like. In a social or mental conflict, it might be loss of willpower, composure, or emotional control...

As attacks inflict stress (which result in consequences), and blocks inhibit stress by increasing defense (resulting in less consequences), the only actual result of removing stress is to extend the conflict. This is the balancing effect to keep in mind. If one side has the ability to remove stress, and extend their staying power in the conflict, and the other one doesn't, you tip the balance in the favor of the side that can remove stress, though the fact that this is a transference in the case of non-rotes does mitigate this concern some.

With that caveat in mind, the first place one could look for an example of what could be done with removing in-combat stress is the FATE SRD. In the FATE SRD (based on Spirit of the Century), there is a skill, Science, which has a stunt Medical Attention [Science].

When using Science as first aid in the middle of a fight, the character must take a full action with a target who’s not trying to do anything else active (i.e., forfeiting his next action). Make a roll against a target of Mediocre; if it succeeds with at least one shift, the subject may remove a checkmark in his one-stress box on his physical stress track. Every two shifts beyond the first improves this effect by one; for example, with five shifts, a character can remove a checkmark in his target’s three-stress box. Success can also be used to “stabilize” someone who has taken a severe or lesser consequence that would appear to be life-threatening (e.g., a Bleeding to Death aspect) – in game terms, this has the effect of limiting the extent to which the aspect can be compelled. A given person can’t be the target of more than one first aid action in an exchange.

Under this approach, one would use an Evocation with the spell's intensity against the target of Mediocre and follow the same guidelines.

Another approach one could use is to look at stress as taken as an intensity of the blow, especially since each dot of stress is an increasing value, and the lower levels of stress won't help with higher intensity wounds. So if your Biomancer wanted to give someone the ability to clear their 4 stress box, they would require an effect with a 4 intensity. Knowing that, you can craft an evocation spell against that intensity.

Note that for balance's sake, and because this isn't explicitly laid out in the rules, the intensity might have to be altered, especially given that a moderate consequence removes 4 stress from the hit, and a mild removes 2, so the equivalency might be a bit different. But I think that these general guidelines would fit the spirit of the rules.