Dorothy,
There are two issues: admissions and transferability of credits. For admissions it is looked upon favorably assuming the student has done well in the courses. What colleges want to see is that students have made the best of the options available to them. They need to show they sought out challenge and did well with that challenge. There is not one way to do this, but lots of ways and sure early college classes are one good path. So, I see benefits for admissions and no downsides.
If your student is looking at highly selective colleges, I would start from the assumption that the dual enrollment credits will not be accepted for transfer credits. Generally this level of colleges will consider courses that were taken for high school credit to be high school work and not equivalent of the level of difficulty of comparable courses at their college. So, at these types of schools your student will likely be still starting as a freshman. They may be willing to take some of the credits for placement purposes (like entering into Calc III instead of repeating Calc II). You may get lucky and some credits will be accepted, but I'd consider that a bonus not something to expect.
Of course your student will always have the option of staying at the local university if they are enjoying it and want to finish college earlier. The other thing to always remember is that it is not just what gets a kid into college, but what makes them successful there. Having a happy high school experience is important - if dual enrollment makes a kid happy that's worth a lot. Also, students who have taken dual enrollment classes or taken on other challenging options have a chance to work on the skills it takes to be really successful at college.
