Tell your friend he wasn't at risk for HIV transmission. Also, ask him to refrain from wrongdoing, if this by any chance involves an underage or molestation in any way.
No, hiv can not be transmitted through a swim suit. The only ways adults get HIV is to have unprotected vaginal or anal sex or share IV drug needles. Transmission takes place inside the body. Air and saliva inactivate the virus. Giving oral is not a risk, therefore. And through the barrier of material? Absolutely no risk.