Yes: assuming they're naturally good in the same way. It's perfectly possible for one person to be a natural linguist through an intuitive 'hearing the language spoken' approach, and another picking it up very easily through an 'explicitly understanding the underlying grammar' approach. Either teaching the other would be disaster if they didn't constantly bear in mind that people learn in different ways.

Whereas a slow learner CAN still apply their own bad, slow way of learning to all and sundry (I've definitely seen it done), but I think is more likely to at least teach in a way that slowly but surely makes sense, and might have tried a wider range of tricks due to not picking it up so quickly.