1. "Don't beat a dead horse"
Your friend is leaving. This is a very personal and often difficult decision. While it is appropriate to say that you'll miss them and that you're sad to see them go, one or two comments to this effect at most is alright. More than that, and you start to sound like you're begging them to stay. I know it's terribly sad, but try to keep it to yourself.
2. Don't immediately ask them what they're selling/if you can buy their wardrobe
If you're really close (read: related) then that's okay, but otherwise, wait until they post a sales thread on the egl_comm_sales. I think it's okay if you purchase things from your friend, just don't rush them to sell off their stuff. I know that even when I sell something that I've never worn, I feel a little sad about it. Imagine how you'd feel about selling your whole wardrobe. Even if they've simply fallen out of love with the style, those pieces contain memories.
3. Ask them why they are leaving, but don't try to convince them to stay
This goes back to the first point. If your friend is leaving, it's perfectly fine to ask them why. It can be a tough choice to leave, and part of being a good friend is listening to what they have to say. People leave the style for a myriad of reasons. Maybe it's something little that can be fixed, such as a misunderstanding. Many times it's simply that they've lost interest in the style. If they're leaving it for a different style, then ask them about the new one (especially if you don't have any idea as to what they're talking about).
4. Stay in touch
If you have a cool community, they may be willing to accept all sorts of J-fashions at their meet ups. So if your friend switched styles, they may still be able to participate in meet ups. If they're not switching styles or your group doesn't let other styles in or what have you (I hope that made sense), then by all means, stay in touch! Exchange phone numbers, have lunch sometime, email, chat online, whatever you'd like. Just because you don't have lolita fashion in common anymore doesn't mean you can't be friends.
I hope this never happens to any of you. But if it does, try not to think of it as the end of the world. Life will go on. People come and go, it's a part of life. I've had friends move away, leave the fashion, and simply stop coming to meet-ups and posting to our community. It's sad and you miss them, but life goes on. New people join up and you can make new friends. I hope this has made sense and helped out with this tricky situation.
As some (hopefully most) of you have noticed, there's a new poll feature up at the top of the page. This is very important! Please vote in it! I have some ideas for new content, and I want to gauge interest in certain things. So you have a little over a month to vote. I can't really offer you anything for voting. All I can say is that by voting, you're helping me come up with new content.