I would suggest that you can't be certain that they won't eventually make the grade and then be a good pilot, (whatever that is). Some people just take a long time to learn new skills, but often they are then as good, if not better than faster learners.
I think all you can say is that it is clear that it is going to take them a lot longer, and therefore a lot more money to get a licence. Are they willing to invest a lot more time and money to learn to fly?
The people who worry me are those, who just don't seem responsible enough, although they are often quick learners.