I don't know how reliable Rome2Rio's driving-time estimates are, but it gives about 7 hours for each of those two legs. I guess one factor is whether a-norton expects to do all the Dubrovnik sightseeing after catching up with the friends. That would allow a bit of time for stops along the way without cutting Dubrovnik short. But there might not be much time for Zagreb itself, which I also like a lot. In addition to the very attractive old town, it has some quite nice museums.
I crossed a fair number of Balkan borders by bus last summer, and generally didn't see significant traffic tie-ups. But there was one crossing, I think between Slovenia and Croatia in Istria, where I observed a long line of cars heading to or from beach resorts. That happened on a weekend in August. I imagine that April would not be so risky unless the refugee situation is slowing things down a lot in that area.
If it's not raining and the weather is reasonably warm, April could also be a good time for Plitvice Lakes. The park was so overrun when I was there late last August that I was glad it wasn't my first trip. It was like fighting your way through the Louvre crowds to see the Mona Lisa, only outdoors. I'd skip Plitvice in bad weather if I had only about 6 days to get from Zagreb to Dubrovnik.
Zadar's another possible stop in addition to Split. The historic district is on a peninsula. The city is a manageable size, not the sort of place where you'd get really frustrated about spending less than 3 days there.
A quick trip to an island would be wonderful, but I'd rough out my itinerary first, to see whether there was any time to spare.
