Praise be to Allah.
It is not stipulated that sa‘i should be done straight after tawaf, according to the view of the majority of fuqaha’. So if someone does tawaf then delays sa‘i for a day or more, his sa‘i is valid.
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (10/240): It is not essential that sa‘i should be done straight after tawaf.
Imam Ahmad said: There is nothing wrong with delaying sa‘i in order to rest, or until afternoon.
‘Ata’ and al-Hasan did not see anything wrong with someone circumambulating the Ka‘bah at the beginning of the day and delaying [sa‘i between] as-Safa and al-Marwah until afternoon. Al-Qaasim and Sa‘eed ibn Jubayr did that. End quote.
See: al-Majmoo‘ (8/99); al-Mawsoo‘ah al-Fiqhiyyah (25/18); and the answer to question no. 109320 .
Based on that, this woman’s ‘umrah is valid, and it does not matter that she went to Madinah between tawaf and sa‘i.
And Allah knows best.