It depends on whether the company's attitude is begrudging or not. If not, there are plenty of straightforward processes that can go in place.

- First is, where possible, you get the replacement in on a defined temporary cover contract, and do a proper handover of duties to them.

- Second, you offer the person on leave paid "Keep in Touch" days that allow the person to stay abreast of what's happening.

- Thirdly, when they return you have a formal hand-back session that brings the person up to speed and makes it clear to all their contacts that they have returned to their responsibilities and taken back over their old position.

I have seen this work correctly and smoothly multiple times, for both maternity and long-term sick leave. It doesn't involve pretending that they are still in post while they are out, but it involves the company being willing to pay more than lip service to supporting them during a period of long absence.