Praise be to Allah.
It is obligatory to form the intention from the night before for every obligatory fast. This is the view of the majority of scholars, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever does not intend to fast before dawn, there is no fast for him.” Narrated by Abu Dawood (2454), at-Tirmidhi (730) and an-Nasaa’i (2331). According to a version narrated by an-Nasaa’i, “Whoever does not form the intention to fast from the night before, before dawn, there is no fast for him.” This hadith was classed as saheeh by al-Albaani in Saheeh Abi Dawood.
At-Tirmidhi (may Allah have mercy on him) said: What this means, according to some of the scholars, is that there is no fast for the one who did not form the intention to fast before the break of dawn in Ramadan, or when making up a missed Ramadan fast, or when fasting in fulfilment of a vow. If he did not intend to fast from the night before, then his fast is not valid. In the case of a voluntary fast, however, it is permissible for him to form the intention to fast after dawn has broken. This is the view of ash-Shaafa‘i, Ahmad and Ishaaq. End quote.
Ibn Qudaamah (may Allah have mercy on him) said: If the first is obligatory, such as a Ramadan fast, whether it is done on time or made up later, or a fast in fulfilment of a vow or expiation, it is stipulated that the intention be formed the night before according to our imam, Maalik and ash-Shaafa‘i. … Then he quoted as evidence the hadith mentioned above.
End quote from al-Mughni (3/109).
Imam Abu Haneefah (may Allah have mercy on him) differed from the majority of scholars regarding this matter, and he deemed it permissible to form the intention for some types of obligatory fast during the day, but he agreed with them concerning the fact that fasts done to make up for missed Ramadan fasts are not valid unless the intention is formed the night before. In fact, some scholars narrated that there was consensus in the Hanafi madhhab concerning that.
Al-Kaasaani al-Hanafi (may Allah have mercy on him) said in Badaa’i‘ as-Sanaa’i‘ (2/585):
The best with regard to all types of fast is to form the intention when dawn is breaking, if that is possible, or from the night before… If the individual forms the intention after the break of dawn, then if the fast is one that is being done to make up for an owed fast, it is not valid according to scholarly consensus. End quote.
He explained what they meant by an owed fast by noting (2/584): It is a fast that is done to make up for a missed obligatory fast, or expiation, or in fulfilment of a vow. End quote.
See also: Radd al-Muhtir by Ibn ‘Aabideen (2/380):
For more information, please see the answer to question no. 192428 .
Based on that, the fasts that your friend did to make up for missed Ramadan fasts with an intention formed during the day are not valid according to the majority of scholars. So she has to repeat the fasts of those days, but she does not have to offer expiation, as has been explained previously in the answer to question no. 26865 .
This ruling to repeat her fasts applies to making up the fasts from last year, if there is still time to do that.
With regard to previous years, some scholars, such as Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him), favoured the view that the one who does an act of worship in a wrong manner, but he was unaware of the right way to do it, and the time for it has ended, does not have to repeat it. We have quoted his words in the answer to question no. 150069 .
If your friend follows this scholarly view, then we hope that there will be no blame on her.
And Allah knows best.