Firstly, why does it matter whether he is a stranger or not? Really ask yourself this. Do you think people you know are somehow less risky in terms of STDs? Surely, this issue would be an issue regardless of how well you know the person. It's about the fact that his status is unknown, isn't it? It isn't about the fact he isn't from your clique and therefore might be germy?
It's a bit vague, but from what I can gather, she has used condoms with him for everything except oral. The risk of contracting an STD orally is relatively low. If you are worried about herpes transmission with barrier contraceptive use, then you should also require your husband to be tested for HSV1+2. Or do you think your husband is less likely to have herpes because... he is from your network?
Who you are trying to protect here, primarily, is you, like you should be. I suggest that you look at STDs a little differently. Instead of you aiming to never catch one, aim to have any STD you happen to catch diagnosed as quickly as possible. This means the onus is on you to get tested that little bit more regularly. You can encourage those around you to take a similar stance but you cannot make them. It's best to just worry about you.