He would be best off talking to his own solicitor. There are loads of issues in this scenario.
If he proceeds to transfer shares in the house to his parents, there are CAT and stamp duty issues all of which will very likely make the whole scenario uneconomical anyway. Plus there are all the issues of what happens if he cannot make the repayments - he has exposed his parents to debt. And if gets married and it becomes his family home.............
Maybe, if he cannot borrow the funds, then perhaps he simply cannot afford to do this extension?
mf