Just wondering can my friend do his border run one month earlier than it is due.

He needs to do this as an emergency has cropped up,

Any good advice is much appreciated.

He can stay up to 90 days, so leaving after even 1 day is fine.

(Presuming he has a valid multiple non-immigrant visa)

Since you mention 90 days I assume he has a multiple entry non immigrant visa. Their is no requirement to stay the full 90 days, A person could stay for 1 day leave and re-enter and get another 90 day entry.

He has a Tourist visa.

A tourist visa only gives a 60 day entry that can be extended for 30 days.

He can go out to use another entry from the tourist visa at any time he wants up to the date his permit to stay ends as long as his visa is still valid.

Thank you all for your assistance.