Yep, I held a couple of grudges but luckily they all got resolved. It all depends on the reason for the grudge I guess, and you ve got ask yourself if you actually understand why the person got so upset that forgiveness is out of the question.
Two people I had a grudge on, just kept telling me that I’m wrong to feel that etc etc and that didn’t help me to forgive them for their actions. Pride also played a role on both sides.

I wouldn’t go into the “health” of holding a grudge, because that can be stressful. On the other hand, in my case, it helped me and those involved to grow somewhat.

Hope you’ll find a way to communicate with your mate :)