"Resonance" as such is a very ominous word, because if you ask 10 teachers, you usually get 10 different opinions.
The question would have to be, in that particular case, WHAT was lacking? Was it projection? Was the colour too bright or too dark? Was the sound unfocused?
Some people believe resonance in classical singing to be the ideal balance (chiaroscuro), or indeed a slightly darker tone than we have in contemporary singing. If that's what's missing, lowering the larynx slightly (!), and widening the pharynx might be the way forward, because it "enlarges the tube" and therefore gives a darker, fuller tone close-up. That alone doesn't make the voice project though, and can even make it muffled, although it will sound immensely big in your own head.
If it's projection and focus that are lacking, you are looking for the singer's formant, and that's actually not achieved by just widening - classical and opera singing are so tricky because you use antagonistic muscle groups, and projection actually involves an element of tightening/narrowing the aryepiglottic region (the "Twanger") whilst still keeping the rest of the pharynx wide and the larynx comparably low.
So we probably would have to know what was missing, and what wasn't right with the sound in the first place.