When does the 90-day validity of a NON-O visa in Thailand start: date of issue or date of entry?

Your friend has to post for himself unless your friend is an infant.

When you arrive in Thailand you have 90 days to get to Thailand

The visa is valid for 90 days from the date of issue and if he enters Thailand on any of those 90 days he will get a 90 day entry stamp.

When he get stamped

He has to arrive in thailand within 90 days of the issue date. On arrival He will be given a stamp in his passport showing the date he has to leave by.

Check the date before leaving the counter,mistakes have been made and it's best to check the date.

I was given a date of 60 days by mistake. The immigration officer changed it there and then.

The 90 day non O is only valid for one entry into Thailand

90 days starts when entering Thailand before the validity of his visa.

the visa is valid for one entry into Thailand for 90 days from the issue date. When he enters Thailand, he will get stamped in for a 90-days admitted stay permit, and the visa itself will become invalid for further use

90 days start the day he enters Thailand

He has 90 days in which to enter Thailand and if he does so in that time he will get stamped in for 90 days