The key question is: does she use her own charger at both locations? I am presuming yes.
'Low' mains voltage anywhere in the developed world represents such a small tolerance differential from the standard supply as to be utterly-insignificant when it comes to powering
electronic equipment. Compliance with published norms for voltage/frequency & current is a matter of observance of statute.
Let me put in another way: modern electronic equipment which is powered (or recharged) from mains electricity now invariably uses automatically-adaptive 'switch mode' power supplies, which efficiently-slice the high levels of presented alternating voltage many times each second, followed by smoothing/filtering/rectification to a reduced exact direct voltage. Bulky, hot, heavy transformers are a thing of the past, in almost all consumer contexts.
These 'switch mode' powers supplies are so tolerant of different mains voltages, they will happily produce exactly the right output voltages anywhere in the world with no external adjustment, effectively from 90V all the up to nearly 300V.
Therefore: it cannot even be considered that the same charger used at two different locations would (or could) produce differing results in charging-times as some obscure variation of the mains electricity- it just makes no sense.
So what factors in reality could make a difference to the charging-rates at those two separate locations? These could include:
- increased levels of stored power consumption, emptying the internal device batteries at the same time as they are being refilled. Suppose one location has poorer wifi coverage than the other, and as a consequence is forcing the phone to use public 4G/5G; or equally, vice-versa. There is normally a heavier power consumption penalty when using public Networks.
- random poor connection between the charger lead and the mobile device (especially if using a legacy mini or micro USB, versus USB-C / Firewire). This can give very-odd effects when attempting recharging, including forcing the battery charge management system within the phone to misinterpret the true latent capacity of the battery itself.
- number and type of background software applications concurrently running, largely-unseen (eg multiple tabs within browsers left open, OS update re-re-attempts (as a function of network type being accessed and selected update preferences)
- Battery life nearing end: many older lithium batteries can begin to swell, through absorption of atmospheric vapour/sweat/rain. This will definitely result in charging times that are very unpredictable.