1. If you are already symptomatic you will need an entire course of antibiotics to get rid of it. Using antibiotics to avoid an infection, like taking only one or two pills because you know you will be in contact with someone, only works to prevent the infection, not to fight it once you have it. I do not recommend taking antibiotics this way except for very specific reasons that obviously would have to be discussed with a doctor. I don’t think any doctor will go for it regarding strep.
2. How do you know for sure they have strep? How long have they been sick? How high is their fever? Strep typically has a fever, over 101F and a sore throat that lasts 3+ days. Most colds start with a bad sore throat, then the throat symptoms disappear in 48 hours and the symptoms switch to congestions. Colds have no fever in adults typically, or extremely low “fever” lees than 100F. The strep quick test is fairly innacurate, the regular culture is better.
At this point, if I were you, I would wait three days if you don’t have a fever, and see if you actually have a cold, which you don’t need to go to the doctor to treat. Unless you already have a high fever, then go ahead and get a culture done so you have the result in a couple days and can start meds if you need them. Strep is one of those illnesses you need to take the antibiotics to avoid possible more severe illness secondary to the initial infection.