Their weak-side winger stays high and our attacks keep ending with a 50-yard diagonal against us

I’d treat the weak-side winger as part of your rest-defence picture before the attack finishes. If they stay high, somebody on your attacking side has to know who is responsible for the space they are waiting in. That may be the far full-back staying deeper, a centre-back shifting across, or the 6 positioning to delay the diagonal. The ball-side attackers also need to understand the cost of a careless turnover when both full-backs are high. I’d train transitions where the defending team has a high wide target. The attacking team quickly learns whether their structure can deal with the diagonal after losing the ball. You do not need to stop attacking because one winger stays high. You need enough protection to make their first action predictable or slow. If the same 50-yard diagonal hurts you three times, the issue is no longer surprise. The team has not adjusted its balance around a clearly identified threat.