Just off-topic, sell this idea to a studio or something, I can see this being a strong title for a summer rom com.
On topic: I can give you some advice based on experience but to precede it I want to tell you this, romance is a fire that burns brightly, it keeps you warm when it's winter and illuminates your life and can reveal to you thoughts and ideas that have always been in the darkest corners of your world. But it always burns out, no matter how much kindle you feed it, no matter what form or shape, even stars and supernovas extinguish and leave an empty black hole. Friendship on the other hand is a rock, at times ugly and heavy but durable and reliable, a rock is a rock and it will never change. 50% of marriages end for one reason or another, but research has shown that the ones that last 'he/she is my best friend' is invariably a common line you hear from those that make it.
I believe friendship is an involuntary reflex, you and your friend will always remain friends. However your friend is right, your relationship WILL change. My sister lived in Mexico for a year and she met the love of her life, but she moved back to Australia and he stayed in Mexico because it's ridiculously difficult (and expensive) to acquire a visa for him. But they were smitten so they tried to have a long distance relationship. They survived off emails, skype and MSN. It didn't work. Their relationship was clearly dying, physical proximity made a humongous difference, and without that it you could almost see a candle flickering in the wind. On top of that the relationship between her and her friends developed a large gap. In that time her friend found a new best friend and my sister felt like she was being replaced. She missed her partner so much as well and it was difficult for her to tell her friend that she would rather have been with him in Mexico than Australia. It wasn't a malicious 'I don't want to be around you' it was just 'there is some place I'd rather be' mentality, true but some would find that offensive. The gap between her and her friend was inevitable and it hurt them both quite horribly. They reconciled somewhat, but they're not nearly as close as they use to be. Earlier this year she moved back to Mexico and now they're doing fine. But Mexico is no place to live, especially for someone who has a Western Standard of living. The police is corrupt, the pay is pathetic, the education is almost non-existent and crime is insanely high. So we're going to see where it's going.
I'm not going to give you any advice, this is just a story, it might be true of many or it might be true for only her, but take from it what you will. I suppose at the end of it the question is if you do go, and if it does end and it does cause a rift between you and your friend, is the experience worth having your heart broken over?