Well ... it goes like this...
I had written an email to Thai because I wanted to find out the dimensions of carry on luggage mainly cos I didnt know which case to take, they sent me back an email stating exactly what it says on their website, about carry on measurements and their 7kg limit, AND ALSO THAT THEIR CHECKED-IN LIMIT WAS 20KG PER PIECE FOR ECONOMY.
... But when we checked in at Sydney airport we were told by the lady at the check-in counter (I obviously dont remember her name!) that we were actually allowed the 30kg going over and 25 kg coming home. I had told her that I was quite annoyed at that as I had specifically asked Thai all about this, and no-one mentioned it.
So ... going over we obviously had less than 20kg each checked in piece (think they were all about 18) BUT ... coming home we all had over 20kg each piece - my 2 checked in pieces were both about 24kilos and my friends were also, and all was fine - def no excess luggage charged. So ... I was therefore of the opinion that this lady at Sydney check-in was correct, cos if she wasnt then I would have been charged for excess luggage.
So ... as I said I do not have this in writiting, only what I was told at Sydney check-in counter. I def had no problems leaving BKK with 24 kg in each checked in luggage piece.
Hopefully this clarifies it for you - like I said - I wdnt take it as gospel, because I do not have it in writing. If I were you (or your friend) I would take 20kg going over and def ask at check-in what the limits are, at least that way you are safe going over, and can then pack whatever kg they say coming home. And get the persons full name that tells you if its 25kg coming home - just in case you have problems!~!~!
I wd def not give you false or misleading information - ONLY WHAT I HAD BEEN TOLD!

Kitty