Yes it is heart wrenching. But I still think it is worth giving a try if you have the finances. Emphasize the positives. I don't know her actual history, but it could be something like this: Describe your relationship without emphasizing any dependence on you now. This young woman has great resilience. Brought up in an orphanage, now working full time and supporting herself. She has a permanent position (provide letter from employer preferably giving a character reference, future potential in her employment and permission for her annual leave. ) She grew up in Russia and was a friend of the daughter you adopted from the same city x number of years ago. Your daughter misses her friend and you wish to invite XXX to visit for two weeks. Have XXX include an itinerary and example of flights she would take. I don't think the expense of booking a flight makes approval any more likely. Both you and XXX understand the importance of abiding by the rules of the visa.
This is obviously just a rough draft of thoughts, but hope it gives you some ideas for moving forward. I still think an IMM expert would be good insurance. It isn't worth trying it on your own and risking refusal. Only gets harder to overcome refusals next time.