Its very difficult to say / judge if he does not give any details, what equipment he uses
"You dont need a Rolls Royce to drive from A to B".
Also important would be information what style he is mixing / mastering. In electronic music, R&B, dubstep you have to work with deep frequencies. Its challenging to get the room proper for that, in such cases good phones can help. But these cost alone > €1600 and not each headphone is really able to reproduce these frequencies, i.e. LCD-X.
For normal semi-professional requirements (and this seems to be the case here) otherwise
an acoustically optimized room would be out of question, the headphone preamps of RME
interfaces are IMHO sufficient.
If he has very expensive headphones or headphones which require a phones preamp with high output,
then the addition of an ADI-2 Pro could be adviced, if its not loud enough or if listening experience can
be improved by the ADI-2 Pro (to be tested by own rehearsal).
Besides of this the ADI-2 Pro has generally some cool features that you can become used to ...
But even if his room is not acoustically optimized I would not exclude to also use Near field monitors
with a narrow sweet spot so that he could cross check the sound with normal monitors and
also phones to completely exclude the room.
At the end it also depends on skills, the sound materials quality and how much money you can spend on your "hobby".
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13