What is an electrical relay on a bus (vehicle) made of?
Relay contacts are made from a silver alloy and thus oxidize with age but wiped clean from arc when switching. Erosion causes ESR to rise and must be maintained by inspection or measure voltage drop under load and replaced if more than a couple watt loss on small contact surface Vdrop*I=Pd watts

Use fuse ratings and replace with greater capacity relay. If wiring melted ensure corrosion free connections checked by maintenance.

The relay contacts are non toxic but the PVC wire insulation is toxic from chlorinated gas fumes. I would have stopped the bus immediately and turned down accessories or shutoff bus and call for replacement bus, but I’m guessing in excited states he may push for litigation on lack of maintenance. The relay may be sealed type which if burned could also be toxic plastic smoke , so hard to say where/what without more forensic details.