Deciding whether to allow an intern to join while on leave or extend their joining depends on company policies, the intern's role, and availability. Company policies vary; some permit joining during leave, while others don't. If your company allows it, you can grant the intern to join during their leave, Consider the intern's role. If it's critical to company operations, it may be better to extend their joining until they return. Availability is important too. If the intern can only work after their leave, extending their joining makes sense.
Ultimately, weigh these factors and decide what's best for the company and the intern.