What you can get from oral sex:
Gonorrhea, herpes and syphilis.
It is uncommon to get a STD from oral sex but it is possible. testing times are Gonorrhea-1 week throat swab, herpes-4 months, syphilis-6 weeks.
Gonorrhea and herpes are the most common, but it is low risk.
Since you are worried I would get a throat swab done after 1 week to be sure. Hepes is possible but your friend would know if he has herpes so if he was given negative just a month ago I would not worry about that. And syphilis is rare in developed countries and unless he has sex with men I would not worry at all about that.
Something to remember and pass on to your friend...use condoms. He is going to get something if he keeps having unprotected sex.
In addition, he also got the all clear about a month ago... but I have no idea what he could have picked up recently.
How likely is it that I could have picked something up? He didn't *** fully in my mouth, but I'm still very very worried.