Drug users get all sorts of different fevers. You are correct to be worried. Tell her your concerns and be available to help. However, don't be surprised if she doesn't pick up on the offer. Along with this I have had extreme swelling and soreness of the inguinal lymph nodes.
Blood tests show no infection.
I'm on Cipro and cephalexin. The doctor seems to feel it's clamydia or something of that nature, though, I have no other signs of that and have been in a monogamous relationship for 6 years. The combination of drugs that you are on is good for coverage of most organisms including chlamydial infections. However, it is rare for chlamydial infections to produce this sustained fever. If the nodes persist a biopsy may be in order since you are really on a very comprehensive drug regimen.