Are they in two interlocking IVA's or is the IVA in one name only (they don't do joint IVA's).
If they are interlocking they will need to be separated and each party will be responsible for their own IVA. Otherwise, if they are still on reasonable terms, they could carry on making payments into the arrangements as they stand.
They will have to submit new income and expense forms, as that will all change.
Depending upon their new finances, how long they have been in the IVA(s) and payment history, they could ask for payments made to date to be accepted.
Lots of "ifs, buts and maybes" I am afraid and a talk with the IP would be in order.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014