No reason not to be friends for the next year. Keep up meeting with her outside of school if they enjoy playing together. It might be more difficult and they might see each other less, but that might might it easier for them to part when they do move. My son's best friend moved between 4th & 5th grade, we knew it was going to eventually happen and I switched my son's school before first grade. We still go them together often during that year. We even visited once a year later.
Think of it this way, if a kid came to your school that you knew would only be there for a year you would encourage your child to play with them even though you knew they would eventually leave.
