Partners Female friend got naked in front of him

Okay so, my partner has this friend, childhood friend. He’s been friends with her for years (she’s best friends with his sister) he has told me that there’s been times she’s got out the bath or shower and come into his room completely naked and “nothing has happened”. He’s also told me that she can be a little weird with the women he dates. This female friend has travelled back to where we are from and she’s messaged him to get her, he told me he wasn’t gonna get her then couple hours later said he’s going to get her. I get he’s her friend but I’m thinking why not message your best friend (his sister) to get you. I’m not wanting to cause an issue etc but if this happened to you how would you feel