URI/Upper Respiratory Infection Possibility of getting a URI?

I brought home a guinea pig friend for my other guinea pig about two weeks ago. She seemed slightly off (less active than my other pig) so I took her back just in case there was something wrong. Turns out she had a URI (showed no symptoms other than being slightly less active than my other pig). I was told they began treating her with antibiotics rights away, but she died this morning. I immediately went online and found that URIs are extremely contagious. My other pig is showing zero symptoms, eating and drinking normally, squeaking like usual (actually more so since her friend is gone) and is still being very active. My vet isn't open until Monday, but I found antibiotics I could order online and get by tomorrow morning (baytril). What do I do? My other pig was just at the vet for a spay surgery but other than that was in complete health. I was able to get a hold of one vet who told me that any vet would just prescribe antibiotics as a preventative measure, but I'm also reading that that can be really bad for them too if it turns out there is nothing wrong. I am so, so scared. I can't lose them both, but I don't know what to do. Do I wait to get antibiotics until she shows symptoms? If she does at all... I'm so scared because my other pig was only slightly less active, but ended up dying even though I'm sure I caught it very early. Please help. I have been reading these forums and you are all so knowledgeable, I need your help and guidance. I am so terrified. I love my little girl so much, I can't lose her too...