@cwilbur – Some things are simple. Penicillin does interact with some drugs as per http://www.drugs.com/drug-interactions/penicillin_d00116.html but not with typical over-the-counter cold medications. She has already said that a) she is taking nothing else and b) she has two different compounds in the house presumably for her own use.
Now if she were taking blood thinners, a second antibiotic, or was undergoing chemotherapy, that would be different. But she says she isn’t.
If she is taking oral contraceptives then her risk of unwanted pregnancy would be fractionally higher, but since she has strep throat (very contagious) and feels lousy she shouldn’t be having sex anyway.
And finally, I have lived long enough that I have run into this particular situation many, many, many times as the caretaker for a disabled husband and two children. If it were more complicated I would say call the doctor. But it isn’t.
Common sense applies in this case.