Yes, you are right that he has nowhere to accelerate in ground effect, so if he’s close to performance limits a prompt transition is warranted. I’d still question is whether the safest maneuver needs to be quite so sporty.
You make a good point about the direction of the turn. It’s a Squirrel, in which the main rotor turns the opposite way to non-French helicopters, so the right turn is indeed increasing tail rotor torque. If he’s close to performance limits he should turn the other way, terrain permitting.
If the location in the video caption is correct (Julius Payer hut, Tyrolian Alps), the actual altitude is 9908 feet/3020 m.
Off on a tangent:
I saw this instructional video a couple of months ago, explaining the concept of overpitching and how to safely attempt a high-altitude landing in mountainous terrain.
Short version: Hover requires more power than forward flight. At high altitude, you need greater angle of attack on the main rotor blades to maintain a given amount of lift, which causes more rotor drag and therefore requires more engine power. At some point - too much weight and too much altitude - there isn’t enough power to maintain rotor RPM, and the rotor will slow and you will descend. Want to land safely at high altitude? Attempt to establish a hover near your landing site, in a location where you are out of ground effect AND can safely descend and transition into forward flight if there’s not enough power for a hover. If there is enough power for a stable hover while you’re out of ground effect, then you know there will be enough when you are in ground effect, so you can move over and follow through with your landing.
I don’t think a conventional* fixed-wing plane can fly on its side, in a straight line, sustainably either - wings, fixed or rotary, are designed for creating lift in a direction generally perpendicular to their plane - and whilst it is probably possible with a fixed wing craft to angle the control surfaces so as to ‘dive’ and tip the plane without banking around, there’s nothing providing actual lift against gravity, so I don’t think it’s going to be sustainable.
*Maybe in some military and stunt planes where the power of the engines makes the wings not entirely necessary to stay in the air.
Aircraft may gain lift from their body in addition to their wings. In a 90 degree bank, as I think has already been mentioned, the wings do not contribute lift against gravity. (They may provide lift, but in a 90 degree bank, none of that lift is in the Z-axis, against gravity. It will help them turn tighter however.)
With rudder manipulation though in that 90 degree bank, the aircraft body can be inclined sufficiently to provide some degree of lift against gravity. This is not as efficient as letting the wings lift the aircraft, but it does allow aerobatic aircraft to maintain a true 90 degree bank attitude and not lose altitude. It helps when the engine is powerful, as flying sideways with a slight pitch with respect to the horizon can generate a lot of drag, which will require a lot of thrust to counteract. If the aircraft is light, the amount of lift required to counteract gravitational force is smaller.
If the aircraft was not inclined in such a way to generate a relative wind with respect to the ground, and thereby lift, then no matter how powerful the engine was, the aircraft would sink. You always need lift to counteract gravity: sometimes the local air mass provides it (thermals, soaring), sometimes Bernoulli does it through the shape of the wings, and sometimes it happens by air rebounding off a surface inclined with respect to the direction of travel.
In general, neither fixed wing nor rotor wing aircraft can sustained sideways flight. If you look closely, even military jet aircraft at air shows pull up slightly before making a “knife edge” pass. This lofts the aircraft while it is turned sideways with wings producing no lift. However this is not sustainable and is carefully calculated to give the impression it is flying at a 90 degree bank for a few seconds.