When you mention 'hi res player', are you thinking that would help to improve how loud they can go?
I don't know what she is using them with that is not providing enough volume, so a player with a decent headphone output may help. A headphone amp would certainly help to boost the volume, but I don't know how practical it would be to add one in. Really depends on what the headphones are being used with.
As for being able to hear herself; I assume she means that the headphones are not isolating outside noise well enough?
DT770 headphones are pretty damn good for that, so if they are not doing the job well enough, then the choice of headphones that isolate better, gets smaller. If she is after full size over ear (circum aural) headphones, then I hazard a guess that only good noise cancellation headphones would be an improvement. They come with a much greater cost though.
One thing that could be a factor and would likely lead to the DT770's not isolating outside noise well enough, is the lack of volume. Lower the volume, the greater the chance of hearing outside noise.
An alternative, if she doesn't mind smaller on ear (supra aural) headphones, is Sennheiser HD25's. They are very good for noise isolation. I'd imagine it's close between the HD25 and DT770 from purely an isolation perspective. Some people say DT770 are the best for that, some say HD25.