You have my sympathies, everything feels grim this time of year - your DD being 15 is a tricky age too as their school work really steps up plus other commitments (social/hobbies/part time job/boy or girlfriend) which can make finding time for the pony hard - on the plus side however if she wants to commit to keeping him she can soon do more of the care independently (on our yard they can ride etc without supervision from 16). I do think a move to somewhere closer so your DD could cycle or bus to the yard without you having to ferry her would be ideal - and a winter break isn't a bad thought although for me it depends on your turnout situation, if he can live out 24/7 then that's ideal for giving him time off, if he has to be stabled for a significant part of the day with poor quality turnout he may need more exercise for his welfare - but this could be ground work or long lining or similar if you don't want to ride?
I think a serious conversation with your DD about involving her in the plans for the future is called for, at 15 she is (hopefully) mature enough to be involved in the decision making rather than just getting to be passively devastated without being part of the solution - she presumably understands how much work and money it costs to keep the pony, how much of that is she able realistically to take on herself either contributing financially by getting a Saturday job when she's 16 or practically by doing more of the work herself? You might need gently though to take the decision for her that that will have too much impact on her alongside studies and having a wider life as well, she doesn't need her own pony to stay involved in horsey life, she could have lessons and maybe help out at a local riding school to keep her hand in without the commitment of her own horse. That being said if the child is really committed having horses through the school years can be a wonderful stress reliever and teaches them a lot of life skills as well so if she really wants to keep going with the loan pony I'd do my best to facilitate that...