I want to say that "as A becomes closer friends with C, B gets jealous and A and B drift apart". What I have so far is A-さんは C-さんと仲良くすればするほど、B-さんがうらやましくなって、A-さんと_____てしまう。
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BTW Bさんがうらやましい may in most cases mean someone (perhaps A in this sentence) being jealous of B. So, to indicate B's jealousy explicitly, you had better use a verb, うらやむ or [嫉妬]{しっと}する. I think Bさんが嫉妬して fits best here. – isayamag Mar 18 '15 at 0:04
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「距離{きょり}を置{お}く」
would fit this context.
If the separation is more passive, rather than intentional/active, then
「離{はな}れ離{ばな}れになる」
These are expressions for drifting apart. "Losing/ending a friendship" has much different expressions.
「縁{えん}を切{き}る」 「関係{かんけい}を損{そこ}なう・失{うし}う」 「～はもうおしまい」
