Even if one of you had HIV, the other was not remotely at risk for HIV transmission in a
spa. If the
spa is properly maintained with disinfectant (chlorine, bromine, silver ions etc), HIV would be instantaneously killed. Even with disinfection, within minutes (let alone 24 hours) it would be much too diluted to infect someone else. Not to mention the water continually circulating through the filter. And I would guess there's no reason to suspect your friend has HIV anyway.