A UTI and STD would both be treated with an antibiotic and it is likely that it would be the same antibiotic for both conditions. Some antibiotics are effective for many bacteria because they are broad spectrum antibiotics and will kill many types of bacteria. When a doctor does not know exactly what is causing an infection they will generally use a broad spectrum antibiotic to treat the infection.

Your friend may have had a UTI. Burning upon urination is a classic symptom of a UTI. If she had a discharge that was discolored, had a foul odor, and caused pain or discomfort then the problem could be an STD. The doctor could test her for that.

Using a condom will prevent the transfer of any STD. If the relationship is monogomous and both partners have been tested and are clean then using a condom is a matter of preference and trust.

Tell your friend to contact the treating physician and ask what the antibiotic was prescribed for. The nurse in the doctor's office should be able to provide this information. She can ask if a culture was done to determine exactly what microbe was causing the infection. This would reveal whether the infection was due to a UTI or an STD. That information would be in her chart. She may have to go in person to get this information due to confidentiality issues.

I hope this helps. If not PM me and I will try to clarify.