Doctor Girl,
It seems like actually 2 issues here. I'm not sure which one you are asking about so I will try to answer both.
Issue #1: How hard is it to get into your state medical school if you went there for undergad?
From some of your other posts it seems like perhaps you are thinking of attending school in Washington State. If so, that may explain what your advisor said. UW School of Medicine has a great reputation and is one of only 2 med schools in the Northwest. What this means is that there is a lot of competition for a relatively small number of spots. I am not sure if they give preference to applicants who did undergrad at UW, but there are a large number of UW premeds, perhaps too many for them all to be accepted there. In general, it is more difficult to get into UW (the med school, not the undergrad) than many other state schools due to its reputation and the large number of applicants. By the way, if you are not a resident of Washington state it is very unlikely that you will get an interview there.
Issue #2: How hard is it to get a residency in your home state if you went there for medical school?
Going to residency from medical school is a different story. In general, students tend to have an advantage when applying to a residency affiliated with their med school. My understanding from friends at UW is that it is not an exception to this. However, getting into residency is highly dependent upon the specialty for which you are applying.
When you say "advisor," are you talking about a pre-med advisor who has worked with a large number of students, most of whom have been accepted into medical school? If not, I would consider talking to someone who meets those criteria.
Hope this helps!