Not sure if such feature would be easy (or possible in current architecture) to implement. But for starters it would be nice (and fairly easy) to allow to configure Daedalus to not create new address on each outgoing transaction. Then Daedalus would send money back to the same address from where initial output was taken.
In that case Martin could give Bob the main address of his wallet and Bob could monitor it thru standard explorer. It’s not the same, as it would require Martin to maintain the wallet in a special way, in which he maybe wouldn’t want to maintain it under normal circumstances. But we should consider the fact that such a use-case is not that standard and popular too.
This would be fairly optimal workaround, and very nice in terms of cost\profit ratio.