Is it permissible to return a worshipper who has inadvertently left the row during prayer to fill the resulting gap?

There is no harm in holding the worshiper and returning him to the row, as it is a slight movement for a beneficial purpose. Similar to this is pulling an individual who is praying alone behind the row to join him. The worshiper should be compliant in straightening the row and closing gaps, because the Prophet, peace and blessings be upon him, said: "And be soft with the hands of your brothers."