The list of bank accounts Ethel's post refers to is probably out of date. In any case, only 3 on the list (Co-op; Barclays; Nationwide) normally provided accounts for people who are actually bankrupt, although you might be lucky & get one of the others to do so. Nationwide has recently changed its policy & is no longer providing accounts for bankrupts so your friend would need to open one with Co-op or Barclays providing he does not owe any money to them.
If he can't get such a basic account he would have to have his wages paid into an account in his partner's sole name, or open a building society passbook based savings account (some of these do allow wages & benefits to be paid in, but DDs or SOs are not possible).
The joint account will be frozen if his name is still on it when he goes bankrupt. Even if he takes his name off beforehand, the fact that he had it will have to go on the bankruptcy form & the Official Receiver will enquire closely into the ownership of the money in the account.
It is not the case that this will automatically be taken - it will be necessary for him & his partner to show who owns what of the money in the account - i.e. who paid it in & from what source. If it was all paid in by the partner, e.g. from her wages or benefits, then it cannot be taken as it is her money, not his.