Like others, explain the situation clearly, but unlike others are suggesting, don't jump to the fork button. Instead, get on a call with them. In the end, you will have to decide if you want to keep accepting their work. Best to talk it through with a non-text medium, video is best, more than half of your communication is lost by not using it.

Also, get over the "I might hurt their feelings" and think more like "I might have to fire this person". On the plus side, you are not impacting their finances or family, just take care to explain your position clearly, oh, and listen first.

Sharing the repo link here might help us help you better.