Can you give a cat Benadryl?

If it's a true URI, benadryl probably won't work anyway. Benadryl is an antihistamine and more helpful for Allergies. You'll need antibiotics most likely, in which case you need a vet to prescribe. You can always look for an emergency vet clinic or just wait until your vet's office is open (assuming it's just after hours and they'll be open the next day). URI's can become serious and sometimes be a sign of something fatal(Felv, FIV, FIP), but it's not typically instantly fatal.