I've got a friend who got a non-immigrant 'O' visa (marriage) in the UK in October last year.

The visa will expire in about 2 weeks from now.

So far he has been to Mae Sai 3 times (every 3 months).

Am I right in thinking that he can go to Mae Sai one more time and get another 3 month stamp (so in effect he will have had 15 months),

before he has to renew / reapply for the visa?

Yup, a border hop BEFORE the 'enter before' (some visas say 'utilized before') date will net another 90 day stay.

Although some have reported success when entering on the actual 'enter before' date it would not be wise to rely on a border run on that date.

why not go to Immigration (if he has the funds) and get an Extension (single), then he will only have to do 90days reporting at the Immigration office and not do border runs.