What must a sister who missed some days of Ramadan fast to make them up, and how can she be guided back to committing to prayer and fasting after her return from 'Umrah?

The girl should be advised and informed about the danger of abandoning prayer, and that it is one of the gravest sins, reaching the level of major disbelief according to some scholars, and that worship is obligatory at all times. She should make up the days she missed from Ramadan before the next Ramadan begins. If she does not respond, she should be boycotted if that would deter her. The utmost efforts should be made to guide her, and if these efforts are expended, then one's responsibility is absolved.