What would you do if one of your staff members got a good opportunity in another company and wanted to leave?

I have had the experience of working in two industries where attrition is the highest -- IT and Retail.
Perhaps because of that, my view towards this has been a bit different.
I have accepted the fact that people will move on.
And thus, right from the day a person joins my team, I start creating a backup for him/her.
In some cases the backup is me, while in other cases the backup is someone else.
Which means, at any given point in time, if someone decides to leave, I would be having some backup in place.
However, despite this I have had surprises.
And the only way to deal with such situations is to request for some extra notice period and ensure that a handover-takeover happens.
And be personally involved in that so that I am the backup in case of a repeat in future.

In a world where the "War on Talent" became wide open any team leader needs to keep his/her talents well protected and taken care of within the organization.
You will be surprised that it’s academically proven that people don’t leave jobs, but rather leave managers.
In other words, it’s also proven that among the top main reasons of why people leave their jobs money comes in a very late stage.
Instead, the relationship with the direct supervisor and its related factors are always occupying the top reasons on the list.
Having said that, the solution is to bring in the talent and have a very transparent communication with him/her.
But, remember that any attempt to make him/her speak up but to end up under the same supervisor will always fail.
Don’t expect him to talk and complaint about his manager then go back work with him and feel safe.
Never works.
Instead, first thing and before any other conversation tell him/her that s/he will be moved out to a new challenge under a new boss…and then you can speak about anything in the world.
Even about the skies are blue and the grass is green.
Once that becomes clear you will be surprised from the reaction.
Much better maneuver than stimulations by promising a promotion or a salary increase.
People leave jobs because they have problems today, not by the valuation time.
A) they don’t believe promises, b) their main problem again is the job/boss itself/himself.
So, money and promotion temptations are just tranquilizers.
If s/he accept to take a tit for tat make sure s/he will still resign again in the near future and you end up not solving the problem, but inflating it.
Wish you all the best.

In professional institutions it's a well observed norm that talented individuals do get lucrative job offers from competitors and upcoming companies.
The most important factor to focus on here becomes that individual talent needs to be recognised, nurtured, motivated and promoted.
When an individual sees growth, feels connected to an organisation and feels that his/her talent will be fostered to the fullest chances of switchng jobs reduce to an enormous amount.
Why would an individual leave then? If a better remuneration is the only factor that drives an individual, then that employee (for whatever reasons he/she may have) has a short-term career approach.
Unless it's impossible to find a substitute, that employee is much better to let go of.

1- You need to assign an employee (A) who will take over the tasks or activities assigned to the moving employee (B).

2- All the task / activities must be documented with latest contacts details if any.

3- Depending on the volume of work or tasks that was handled by employee (B) you might need to have a transition period just to make sure that employee (A) is fully cabable to do the new tasks.

4- You need to announce to the clients/customers which were served by employee B, that now employee A is handling their business.
I hope that helped you