I agree with dkh.  If they can pay everything off by the extended deadline I wouldn't fool with an installment plan.  I'd suggest they make an extension payment on IRS Direct Pay, then make Balance Due payments as they get the money in (interest runs until the payment is made so the sooner the better).  Once the tax is paid off, file the return and the IRS will send them a bill for the interest & late payment penalties.

Or if they move to CA before filing they'll have a disaster zip code and the IRS may just ignore the interest.