Fulfil it on her behalf. (Using translation from Muslim 1638a)  

أَنَّ سَعْدَ بْنَ عُبَادَةَ اسْتَفْتَى رَسُولَ اللهِ ﷺ فِي نَذْرٍ كَانَ عَلَى أُمِّهِ تُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ فَقَالَ «اقْضِهِ عَنْهَا»  

"Sa'd consulted the Messenger of Allah about a vow which his mother had to fulfill, but she died before doing so. The Messenger of Allah said: 'Fulfill it on her behalf.'" (Using translation from Nasāʾī 3659)  

اسْتَفْتَى سَعْدُ بْنُ عُبَادَةَ رَسُولَ اللهِ ﷺ فِي نَذْرٍ كَانَ عَلَى أُمِّهِ فَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ فَقَالَ رَسُولُ اللهِ ﷺ «اقْضِهِ عَنْهَا»  

Sa'd bin 'Ubadah came to the Prophet and said: "My mother has died and she had a vow to fulfill but she did not do so." He said: "Fulfill it on her behalf." (Using translation from Nasāʾī 3663)  

جَاءَ سَعْدُ بْنُ عِبَادَةَ إِلَى النَّبِيِّ ﷺ فَقَالَ إِنَّ أُمِّي مَاتَتْ وَعَلَيْهَا نَذْرٌ لَمْ تَقْضِهِ قَالَ «اقْضِهِ عَنْهَا»