<p>I have never been a fan of rooming with best friends, but “friends” can be fine. Typical roommate issues can follow you home, it is harder to reinvent yourself with someone who has known you for years. It will not be easier dealing with messy rooms or sexiled just because you are friends with the boyfriend as well. It will be harder to complain about possible roomie squabbles since you did have a choice.
We have a lot of kids from our hometown that room with friends, go to school together, etc and their social circle is smaller. It doesn’t have to happen, but we have seen it a lot in our area.
As a word of caution, two best friends from my school (best friends since first grade) dormed together in college. They were still not speaking to one another at the 25th reunion. It is possible that your grandmother has an anecdote or two that is affecting her judgement. </p>