Those old years will be a joint liability that IRS will collect from either of them.

Any new MFS liability will be a separate liability, but IRS might be unwilling to enter into a new installment agreement if the joint is still outstanding. The client should probably call IRS once they file, if they still owe on this year. 

Going forward, they should each be careful to not get a refund, but also not to owe any more than they can pay.