I'm prone to tendinitis. I've had severe wrist problems (bad computer ergonomics), hip pain (bad squats), shoulders (dips) and most recently ankles (loaded carries).
1) Stop making it worse: stop doing the problem exercise or fix your form.
2) Healing will take many months, so don't just stop working out "while it heals" -- you can't afford to wait that long.
The worst part of tendon problems is how very long they take to heal. Recovery from my wrist problems took years, with many setbacks. At my worst I feared I'd lose my job and career. I couldn't turn doorknobs or pick up grocery bags. But now I can type pain-free and bench, press, power clean, etc.
I had to completely change how I use a computer keyboard. I went to the SS seminar to get my squat fixed. For my shoulders, I just gave up dips entirely. For my ankles, I found a form change for my loaded carries so they don't aggravate my ankles anymore.
Do you really need to do the grip training? Maybe you can safely do a grip exercise that keeps your wrists straight, like farmers carries or one-armed deadlifts instead of using a crusher.