Shaykh Muhammad Ibn Hizaam was asked:
Does a person sin if his brother says to him, “sit in my place so that I may go and take care of some of his affairs”. If he sits in his place, does he sin?
If the place is far and will take long, then it isn’t allowed for him that he reserve the spot from his Muslim brothers in the masjid, I mean in the rows.
It’s not allowed for him to reserve it from other than him, it’s not allowed for him to co-operate with him upon reserving this spot.
“Help you one another in Al-Birr and At-Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression.”
If the place is near and he won’t be in it for long after him, then whether he sits or doesn’t sit, if he returns, then he has more right to it as
the prophet صلى الله عليه و سلم said:
The one who stands from his place of sitting, then returns to it, then he has more right to it
Muslim reported it from Abu Huraira Radiallahu Anhu
Majid Jawed Al-Afghanee