It depends on their priorities, but to maximize the sheer number of attractions in 1 day without duplicating and based on what I've seen for how fast
Genie+ slots go:
Get Genie+ for sure
Start at DL, be there at rope drop, hit any desired non-Genie attractions (particularly Fantasyland) within the first hour
Book Space Mountain as the first Genie+ for 8:15-9:15 ish and use after completing desired non-Genie attractions (use grace period if needed) followed by Buzz and Star Tours (both should be immediate return times early in the morning so they can do all of Tomorrowland in quick succession before 10 am)
Book Matterhorn after tapping in for Star Tours and use at the earliest opportunity
Head to the other side of the park
Book Genie+ based on soonest return times for BTMRR and Splash Mountain and immediately book/do the other (if Indy Genie+ is getting too far out, possibly skip these and plan to book in the 10-12 pm window after DCA closes, as they tend to have good evening availability)
Book Indy Genie+ after hitting those first 6 Genie+ attractions.
This will likely leave a gap in the schedule for lunch/non-Genie+ activities. I would aim to get a ROTR individual lightning lane in this window and ride Smuggler's Run immediately before or after ROTR using the single rider line.
Once the Indy Genie+ selection is used or 2 hours pass, book Guardians of the Galaxy at DCA. This will likely leave another schedule gap for non-Genie rides/activities at either park, including possibly doing single rider for Radiator Springs Racers or Web Slingers or individual lightning lane if desired.
The second Genie+ selection at DCA is a hard choice -- with World of Color closing Incredicoaster early, the window for getting a Genie+ for it in the afternoon/early evening is pretty short, but Midway Mania runs out much more quickly. I would evaluate this choice based on Genie+ return times and posted standby wait.
Once those two are slotted into the schedule, Soarin would be my next-highest Genie+ priority at DCA, followed by Goofy's Sky School and Monsters, Inc (Grizzly River Run has a good single rider, so they could use that if desired).
Plan to return to DL either before 8:45 pm or after 10 pm to avoid trying to go up Main Street during parade/fireworks
Book Haunted Mansion Genie+ for after return to DL
Hit any other desired attractions through standby until the park closes.