is the current inflammation due primarily to a pathogen or is it now a reactive arthritis? - an immune response to the pathogen? If the former is the case then I would be very cautious about applying ANY electro-physical agent - the potential harm being stimulation of microbial reproduction. If it is the latter then this is less problematic. Symptomatic treatments like heat or ice are fine - whatever feels best.
How is the knee being managed medically? If there is active sepsis what is the pathogen - is it treatable by antibiotics? the most common are Neisseria gonorrhoeae (from a sexually transmitted infection, so usually in younger adults) or Staphylococcus aureus - in which case is it a resistant strain - Patient may be in big trouble if is is a highly resistant strain. Streptococcus makes the third most common class of bacteria. However there are also viral and fungal infections - which may have little available medical treatment.
Has the patient been advised to where a splint or to move the joint freely? What about weight bearing in walking?
As for therapy I think I would focus on maintaining or improving muscle strength, particularly in the quadraceps. There is likely to be reflex weakness due to pain and inflammation and joint effusion and subsequent atrophy. Gait probably deserves some focus of attention taking into account splinting and weight bearing.
Probably well worth talking to the patient's rheumatologist or orthopaedic surgeon about management.