SQLFiddle demo if there are records 'a@g.com'
SQLFiddle demo if there are NOT records 'a@g.com'
INSERT INTO users_friends (userId, friendId)
SELECT 77, 100
FROM users WHERE email = 'a@g.com' LIMIT 1;
Another way would be:
INSERT INTO users_friends (userId, friendId)
SELECT 77, 100
FROM dual
WHERE EXISTS
( SELECT * FROM users WHERE email = 'a@g.com' ) ;