I think it ... depends. If the character being healed belongs to the player in question, I would not consider it godmodding if he does not wish his character to be healed.
In which case, it would be acceptable to say the healing attempt failed.
I would agree with CorralDust, if you're trying to heal another players' character and the player doesn't agree then it wouldn't be godmodding at all. Regardless of your characters' abilities you can't force them on someone, with positive or negative result.
RP noob here, but this is pretty interesting to me. Let me put forward a test scenario:
RPer1:
Abba rushes over to Bently, eyes wide with panic as she sees the blood pooling underneath him. She presses her hands against the knife wound, and they start to glow with white light, her healing energies spilling into her injured companion.
Slowly, the wound knits closed, Abba sighing in relief as he stabilizes.
Assuming Bently is RPer2's character, I imagine there are a few distinct points at which they could change the action, and possibilities about how that would effect how god-moddy it is.
This is how I'd consider the breakdown:
If Rper2 was to say OOC that Abba never rushed over to them or tried to heal them, and picked up as if that hadn't happened, that would seem like godmodding to me.
Now, if they were to say that when Abba tried to heal them, it did nothing, that seems like more of a grey area without a little explanation. It seems odd to declare that someone elses powers, that ought to work, do not, for no discernable reason. I think this is pretty easily fixable, just add a reason. perhaps they would veto the last line, "Slowly, the wound knits closed, Abba sighing in relief as he stabilizes." and replace it with "Despite Abba's best efforts, the knife must have been coated in something, and her healing energies can't get the wound to close." This seems a little more reasonable to me.
I think another reasonable cut would be when Abba goes to put her hands on the wound. If the actual character Bently does not wish to be healed, I think it would be kosher for him to bat her hands away or shove her back or something to prevent the attempt entirely.
Again, RP noob here. Am I on the right track with these kinds of resolutions?