That Japanese friend said "I was finally able to enjoy making a friend, but it's unfortunate that I have to go back to Japan at this time."
ご質問ありがとうございます。
「やっと友達と仲良くなってきて楽しくなってきたのに...」はそのまま英語で「 finally able to enjoy making a friend, but」と言います。
また、その日本人の友達は「やっと友達と仲良くなってきて楽しくなってきたのに、もうすぐに日本帰らなといけないのは辛い。」と言っていました場合、そうすると、説明するとき、「That Japanese friend said "I was finally able to enjoy making a friend, but it's unfortunate that I have to go back to Japan at this time."」という風に言えます。
ご参考になれば幸いです。
But I finally was able to get closer to my friend and start having fun...
"やっと友達と仲良くなってきて楽しくなってきたのに..."
- But I finally was able to get closer to my friend(s) and start having fun...
「やっと友達と仲良くなってきて楽しくなってきたのに、もうすぐに日本帰らなといけないのは辛い。」
- I was finally able to get closer with my friends and it was starting to get fun but I have to go back to Japan soon. It's tough.