Having frequent night ejaculations is not itself going to pose a risk to his health, no. Priapism is dangerous because it can lead to clotting and, in a worst-case scenario, gangrene. By contast, virtually all adult males have several erections a night during sleep; and while noctural emissions are most common from teenagerhood through the mid-20s, it is normal for some males through adulthood, and no harm has ever been linked to it (other than, Wikipedia points out, the inconvenience of cleaning up semen).

That said, more frequent noctural emissions can be linked to testosterone usage -- I wonder if the OP's partner is using any sort of testosterone supplement, or has an imbalance on that end.