I would do it with a cord and plug, that way you're not leaving the guy a hardwired cord into a disco with bare ends. I would think your liability would stop at the receptacle, and if he kills himself with something he plugs in thats his problem.
Pin and sleeve is nice, but $$$. 14-60 is expensive too. If just for testing and not running long periods of time I would probably just use a 14-50 if the hot tub is good on a 50a breaker. If I wanted to make it good and there was a budget, I would use pin and sleeve (it would be the orange ones for 125/250v 3p4w) and on the male plug I would use loose 4ga type SC bundled with big adhesive lined heat shrink where they go into the connector's clamp, and install ferrules on the bare wire ends. Even slicker would be a pin and sleeve with the integral disconnect switch but those are a few hundred bucks.
Pin and sleeve is nice, but $$$. 14-60 is expensive too. If just for testing and not running long periods of time I would probably just use a 14-50 if the hot tub is good on a 50a breaker. If I wanted to make it good and there was a budget, I would use pin and sleeve (it would be the orange ones for 125/250v 3p4w) and on the male plug I would use loose 4ga type SC bundled with big adhesive lined heat shrink where they go into the connector's clamp, and install ferrules on the bare wire ends. Even slicker would be a pin and sleeve with the integral disconnect switch but those are a few hundred bucks.