Well, if it is just a safety stop, the buddy should surface as soon as s/he realises they cannot return to the line (it is just a safety stop...), and I'd leave them to it, and finish up my own safety stop. Unless it is absolutely ripping, someone on the surface can throw them a line.
If they don't, I have to assume they have a more significant problem and I would go and try and render assistance.
If a buddy lost contact with the line in a strong current, and they thought the smart thing to do was deploy an SMB and drift off whilst caught in a current too powerful to swim against rather than surfacing immediately, I wouldn't be formed a favourable view of their sense of risk analysis.
If they don't, I have to assume they have a more significant problem and I would go and try and render assistance.
If a buddy lost contact with the line in a strong current, and they thought the smart thing to do was deploy an SMB and drift off whilst caught in a current too powerful to swim against rather than surfacing immediately, I wouldn't be formed a favourable view of their sense of risk analysis.