About an year ago, Bob requested to be transferred to our team because
he was bored and did not like what he was doing. He was promised to be
transferred in a few months, but a year has passed and nothing really
This is the only thing you are sure of: For a year Bob has wanted to switch jobs, he was promised a switch, but nothing happened. ...
Honestly, I think you're asking the wrong question entirely. If Bob's work and knowledge are valued in the company at all, you need to look at the bigger picture and not just in the context of your team.
Bob was promised a transfer, the company failed to deliver. Perhaps his patience is exhausted and he's looking to move to another company that follows ...