Hello Dr. My Fiancee Had An Allergic Reaction To Bactrim.

Hello Dr. My fiancee had an allergic reaction to Bactrim. Yesterday she went to the local ER with a swollen, red face and spots all over her body; from her neck to her lower extremities. The spots were also red and itchy and swollen. She had initially been prescribed bactrim to treat spider bites she sustained on her shins . She was discharged late last night after giving her prednosone (?) and Benadryl IV. Her symptoms have flared up at 11 am PST. We have the prednisone (?), but we only have 25mg benadryl tablets. How much benadryl should I give her to equate the IV dosage given her at the ER?