Once the hypothetical girlfriend has been on antibiotics for 24-48 hrs she is no longer contagious and to be extra careful the imaginary boyfriend could pop one of her antibiotics an hour before snogging.
This isn't as well studied as I'd like, but he might ask his hypothetical girlfriend to gargle a peroxide-based mouthwash at the start of the night (or at regular invervals). He might also want to use mouthwash himself at the end of the night (or at regular intervals).
We know that peroxide mouthwashes kill viruses and bacteria very quickly, and at the very least it should decrease the amount of bacteria you're exposed to and hopefully that would at least decrease severity?
The CDC also thinks it spreads through the air, so if he's hypothetically not kissing his girlfriend, the standard COVID prevention methods might help too (ventillation, air purifier, masks).
Regarding antibiotics, if he isn't able or willing to take antibiotics proactively, he might lookup the incubation period for the disease and schedule a doctor or urgent care appointment for the earliest point where the test they use to confirm bacterial infection is likely to come back positive.
Googling this question tells me that the best way to avoid strep is by washing your hands often. But say, hypothetically, that someone’s girlfriend has told him that she has strep the day before Valentine’s Day. Knowing he’ll be exposed, are there any steps he should take to reduce the chance and severity of infection? (His hypothetical tonsils were removed long ago). Since this person is hypothetical, advice to avoid his girlfriend on Valentine’s Day will not reach him.
For example, for COVID, there are reasons to think that taking Zinc lozenges (https://www.lesswrong.com/posts/5DKqK3hEzzBoGF47C/consider-taking-zinc-every-time-you-travel) and vitamin D (https://astralcodexten.substack.com/p/covidvitamin-d-much-more-than-you ) might help. Strep throat is not a respiratory virus, but rather a bacterial infection, so does the same advice apply? Or are there other things that seem promising to try? Antibiotics are a natural thought, but is taking antibiotics before any sign of infection reasonable? And if so, would a doctor even prescribe preventatively?