The general rule of thumb is to allow about 45 minutes to get anywhere using Disney transportation. To be safe, I'd allow about an hour. I've only had it take an hour to get from one of the Epcot resorts to the MK once, but that was because the bus didn't come for a while (I think there might have been an issue because the CM at the bus stop kept asking when the bus would come and they were told within 5 minutes for about 20 minutes), then the bus made multiple stops and it took almost 20 minutes at one stop to get all of the
ECVs loaded. So all in all, it took about an hour to get to the MK. Otherwise, it usually only takes about 30 minutes or so. Still, I'd allow an hour to be safe.
For dining, I tend to allow between 60-90 minutes for a resort TS meal. At a buffet you can go as fast or slow as you'd like, but I'd still budget about 90 minutes to be safe.
So overall, unless there are major issues, I'd guess they should get into MK before the FP+ window expires.
Alternatively, as a PP mentioned, if they're running late or feeling like they're running late, they can always take at taxi to the CR and then walk to MK. It'll run about $20-$25 plus tip.