#1
My brother's acoustic guitar bridge has come slightly undone from his yamaha acoustic, it seems to be slowly worsening. Is it best to replace it completely or is simply "re-gluing" it ok for a several hundred dollar yamaha? I'm totally new to this specific problem....

Can anyone point me in a direction towards websites or videos showing how to make the repair?
#2
loosen the strings and take it in to a guitar technician. regluing it should be fine. since it's a several hundred dollar yamaha, i'd assume that it is made of solid wood. if that's the case then your guitar may have been under humidified, causing the bridge to lift.

Do not attempt to repair the guitar yourself. You should get a proper analysis of your guitar done in order to pin point the problem and get a proper solution.
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#3
Sounds like the guitar has been left in a car on a hot day. High temp. will melt the glue enough to start the bridge lifting. Then again it could be some other reason. It's a cheap repair if it's only due to failed glue but special clamps are needed to do it properly. It might be another cause than failed glue. Have it checked out by a pro-repairer.

Have a look here:

http://www.frets.com/FretsPages/Musician/Guitar/Maintenance/CrackedBridges/crackedbridges.html