If it’s purely on SQ, you can ask SQ to split into separate booking references (PNRs) and cancel one of them. You pay cancellation fee for that one person.

But if it’s a *A award, they can’t split the PNR unfortunately. In the past I’ve asked them to notate this as a system limitation on their part where they should waive the cancellation fee and no show fee. Then once the other person has completed their flights, you’d have to call again to cancel the unflown ticket - with full miles and taxes refunded without any fees deducted. YMMV though, and may require their CSO to escalate internally for exception handling.