Would you feel comfortable adding benlysta to patient already taking both mycophenolate and tacrolimus (and hydroxychloroquine) who still has some evidence of active lupus nephritis?  

ABSOLUTELY (if the mycophenolate + tacrolimus combination therapy had had positive benefits on decreasing proteinuria plus was well tolerated thus far)!1. Per the package insert label for indications, "BENLYSTA is indicated for patients aged ≥5 with active systemic lupus erythematosus (SLE) or ac...

I would first make sure they are actually taking their MMF. Twice a day medications are hard. Multiple pills twice a day harder. Have you ever checked a level?

This would be the ideal candidate, particularly if still serologically active. The main advantage of adding belimumab in this context is decreasing the likelihood of relapse as induction therapy is tapered, most notably in year 2 and year 3 of management.