The best answer depends on what the goal of the side trip is and how much effort you want to put into it.
The best access at Universal is for guests staying at one of their three on-site resorts, so if seeing WWoHP without a long wait and/or squeezing in all of the big rides at both parks in a single day is important, they should consider staying at Universal.
They could save a few bucks staying at a cheaper off site hotel vs. Univeral or Disney, but if that means double booking when they have a room paid for at Disney already there are no savings and no real advantage other than proximity to Universal. They's also still have to figure out how to get back and forth between Disney and wherever they stay and between wherever they stay and Universal.
Renting a car at WDW would work, obviously, but would require getting to and from where you are staying and the either the S/D or the CCC to get the car. You'd have to check rental car rates to get a sense of the cost.
The other option is a
Mears shuttle between WDW and Universal for $19 per person round trip. That's probably the easiest of all if they just want to do one day at Universal.