The Friesian temperament, honestly assessed
The Friesian's reputation works in your favour: this is a breed known for a calm, willing, people-oriented nature, and many Friesians watch a fireworks display with the same steadiness they bring to a show ring. But a reputation is not a guarantee about your individual horse. A young Friesian meeting his first full display, a horse already unsettled by a yard move or a lost companion, or one that has had a bad experience before can react out of character. Judge the horse in your stable rather than the breed in the book, and make the same preparations you would for any horse, because the cost of preparing a calm horse unnecessarily is an evening's hay, and the cost of the other mistake is much higher.
Why size and colour change the risk
If a Friesian does panic, the consequences scale with him. He is a heavy, powerful horse, and fencing, gates and stable fittings that would hold a lighter breed can fail under his weight when he hits them frightened. The second risk is his colour: a loose black horse on an unlit road at night is close to invisible until a driver is on top of him. Cheap insurance exists for exactly this scenario. If there is any realistic chance he could get out, fit a light-coloured or reflective halter, and consider reflective leg bands, so the horse carries his own visibility with him. Walk his fencing and gates before dark with his size in mind, checking the posts and rails you would normally trust.
The evening setup
Bring him in before dark and keep the night ordinary: deep bedding with banks, a generous amount of hay to occupy him, water topped up, lights on in the stable and yard so flashes do not plunge him between darkness and brilliance, and a radio playing at a normal level. Stable him with his usual companion in sight, because Friesians bond strongly and a settled friend is half the calming plan. Then resist the temptation to consider the job done. A thick-coated breed works up heat when stressed, so check through the evening for sweating under that heavy coat, restlessness or a horse that has gone off his hay, and open ventilation if he is getting hot and bothered.
If he lives out, and through the night
A Friesian that lives out with his herd may genuinely be happier staying out than standing alone in a strange box. If you choose the field, keep the herd together, walk every boundary before dark, shut gates toward the road, and remove anything in the field a galloping horse could hit. Whether he is in or out, visit during the evening rather than only at bedtime: a horse coping at nine may not be coping at eleven when the biggest display starts. If he is injuring himself or cannot settle at all, your vet is the right call for advice.
If the worst happens
The scenario every part of this plan works to prevent is a Friesian loose on dark roads, black against the night. Should he get out anyway, act in the first minutes rather than searching alone and hoping: the map shows his alert from his KinsLe profile, where the Lost Pet Alert System costs nothing and a finder can send you a direct message, with the reflective halter you fitted giving drivers and finders something to see.