I assume that the doctor has cleared her to exercise, both on grounds of RA AND her diabetes.
If it was my client, I would impress weight loss as a top priority and would recommend that she see a dietitian to assist with it so that she will eat optimally to manage any problems with diabetes.
She will be in the difficult situation that she has to balance the limitations that RA may inflict upon her with those to control her diabetes. RA progression can be unpredictable but excess weight will only make matters worse.
You say that she likes running and biking. While these are not ideal exercises for RA (many RA sufferers do much better with aquatic and very low impact exercises), they could help her with weight loss. I would check with the RA doctor whether it would be considered advisable to maintain those (at least biking) or whether she should go to lower intensities.
Light resistance training is certainly a good additional option as tolerated with RA.
This is a tricky situation, and I suggest you work very closely with her healthcare provider.