This is quite interesting. Your score of 7 is correct I believe, as the rules state that the Longest Highway is 'the single longest unbroken line of Highway routes that connects a streak of adjacent spaces'.
Interestingly, the rules for the Red and Blue version continue, '...not counting any loops or branches'. However, the Green and Yellow versions just say, 'not counting any branches'. Those versions then go on to explicitly state, 'You score 1 point for each space that has a part of your single longest Highway path drawn on it. Spaces may be counted
more than once if your line goes through them multiple times'. There is then an example that specifically includes a loop with three spaces counted more than once. (It's worth referrring to that graphic, as it is difficult to explain!)
Your opponent therefore scored 11 - including the loop with the cross-roads counted twice, but excluding the final stretch of Highway on the right.
I learnt something new today
