:( Don't expect her to forget quickly - adults can be very dismissive of small children's feelings and friendships, coming out with trite little phrases like "children are flexible" and "happy mummy happy kids" (blerg) as excuses not to put much thought into how moving house/ country/ entire life affects children BUT that said she will be fine of course!
Instead of just expecting her to forget, make a feature of the friendship - if the other mum is OK with you taking photos then get a few nice pictures of the girls together and make a scrap book - or even one for your DD and one for the friend. Do some special things together, see if the other mum would be up for Skyping, if you'll be moving back to the same area and they are likely to be still there and she'll be going back to the same preschool/ going to the same school plan a special, fun a little "see you in a year" tea party...
Then when you move you can take her lead - they might forget each other quickly but they probably won't for a while (my DD was not quite 2 when we left the UK but I had been hanging out with a small group from antinatal class since maternity leave and spending several days a week together as the babies all turned into toddlers, and she had incredibly good language skills and talked about her friends by name and in detail for a year or so after we left, despite making new friends.)
You aren't saying "goodbye forever" to all your friends, it must be hard for DD if she thinks she is! A year is an eternity, but you can tell her she will be coming back when she is 4.
Frame the move positively but also make a special time of the next 5 weeks and be positive about the friendship and making some memories and keepsakes for her to look back at in the first weeks after the move.