I've recovered from anything guitar playing related thrown at me by taking sufficient time to allow healing. If tendonitis was amongst it I can't say as I'm not a person going for a diagnosis if I think that a rest period is all it needs to heal and it always did heal and never resurfaced.
At the moment I have a blocked nerve somewhere in my shoulder area which makes my left hand feel numb and tingly, it's not helping much when I play but I can still mannage. This obviously is not something which needs to heal in the ordinary sense, the nerve must relocate itself where it's supposed to be but sofar no luck. Still, no matter how long it takes I won't have surgery on it, someday I'll make it come back to normal again by excersising the shoulder and perhaps with infrared and some massaging. Surgery is only needed when things start hampering your life and you can no longer function normally, but it's never been that bad fortunately,
Ludwig