There really isn't a need to scare him into getting tested for std's when the occurrence of std's is not likely. Strep throat is highly contagious and is much much more likely to happen. Std testing is costly. So, if there is not a real reason to believe this is an std, then it isn't necessary. Also, we do not know if sex was involved.
It's possible to get some stds in your mouth but rare...I mean it does sound like strep to me. I had strep once and it sounded the same. I would just tell your friend to get tested for every std and get a strep culture ASAP.
It can be many things and probably not a STD. He can seek a new Dr or follow up and request some tests.