If you/your friend wants to be a Diver in the Royal Marines, SBS is the only specialisation but it's far removed from RN Diver.
Your friend needs to determine which branch appeals most because joining the Royal Marines or the RN Diver branch is not the easiest route to achieve the other and both carry a higher than average rate of failure due to the rigours of training and, as such, are a relatively high risk career strategy.
Decide which you want most because you need to be content with the primary role in case things don't pan out as hoped.
Beware of planning to 'cherry pick' the roles that appeal most from the outset as it'll most likely take a minimum of four years before eligible to transfer and your personal priorities and career direction can change significantly during that period.
If I, or your careers adviser, were simply to declare "Yes, it's been done before" that certainly doesn't mean it's a given in the real world.