Firstly, you have (quite rightly) identified this question as one of density, not mass. But, I think Gravero is over-doing it a little. Let's look at why.
The internal density of the Earth is generally considered to be around 5.5g/cm^3 which is pretty dense when you consider that water is supposed to be 1g/cm^3, and we consider it relatively dense by comparison to a lot of other materials we work with every day.
The Sun on the other hand is estimated to have an internal density of 160g/cm^3, and that's clearly enough to initiate fusion. That means that if you have densities of around 30 times that in the centre of the Earth, then you can get fusion and all other sorts of interesting materials being created.
The thing is, Gravero is reducing all mass in a 2m diameter (?) sphere into a 1mm diameter sphere. That's a compression ratio of 10 orders of magnitude (based on volume of a sphere).
That's so much, you're getting close to the density inside a neutron star. You don't need that, and yes, that's going to cause heat and hair loss issues for sure.
BUT, what if Gravero dials it down a little?
Human density is roughly the same as that of water (for obvious reasons). So, increasing density by approx. 150 times (2 orders of magnitude) generates mass density similar to that in the centre of the sun. The heat being generated will also increase pressure, and you can convert your human opponent into diamond as the water flash steams and you're left with (mostly) carbon under pressure.
What I don't know is how much density and how much pressure in the form of heat is required to make a diamond out of a human body's carbon. But, pressure is essentially a 2-D force, whereas density is a 3-D measure. As such, I'm pretty sure that Gravero doesn't need to go anywhere near 2m diameters to 1mm diameter to generate diamonds as heat and pressure will be produced long before you get down that far.
I'd argue that all Gravero needs to do is increase gravity to a point where internal density is increased 100-fold (possibly a little less), for a short period of time. Yes, there will be heat at that point and the flash-steam generation is really going to do the most damage if you're close, creating steam burns that really can make you bald. But, Gravero has to have experimented with this capability (preferably at a distance) and figured out the right mix of density increase to cause the desired effect without releasing any more heat than necessary.
On the plus side; Magnifique Courbee is in for a real treat; Gravero can after all create a lovely romantic wood fire to spend a winter's night beside and hand over a diamond at the end of it as a souvenir by applying the same approach to firewood.