The boost will include any game which is finished or started through it's duration.
Dunno how the friend (MP game?) and a 27+ holes game (which course?) would work, but two CDP boni can be doubled with one boost (has been posted here before):
1) Play a game (18 holes tournament?) to the last green, be sure that it will invoke the CDP bonus. Buy or gain the XP boost, finish the game => double points, double bonus.
2) Do what you like until 5 mins before the boost expires (single tournament rounds (Par 3) would be my proposal),
3) Start a new single game (18 holes tournament?) shortly before the boost expires, save it in between,
4) Stay away from playing until the next CDP (GMT) day,
5) Continue and finish your game => double points, double CDP bonus again.
It has been executed to 4.000+ XPs for a one-hour boost, add 2.500 (estimated) for four hours time. The only downsides are
- you'd have to avoid playing for 20 hours - would you get more XPs by playing continuously?
- you can only do it this way every two days to capture the CDPs.
I judge it not to be efficient if you'd have to buy it, but worth a thought in this promotion.