A 13 year old girl would be very attracted to 17 year old boy. The question would be why would the boy of 17 be attracted to a 13 year old girl? Is she very mature for her age? The boy may be a person who wants to control and a 13 year old girl would be very easy to control. This would be a warning sign. It would depend on how mature the girl and boy are. A lot of the time what happens is one grows up and the other does not and the relationship dissolves.
Depends what you mean by involved, is she your child as if you think it is wrong you can always curtail it, could you enlarge for us to get a better understanding of the situation.