The friend arriving on THAI with a THAI connection to HKT has two choices:

1. stay airside and transfer to HKT and clear custom and get bags on arrival at HKT (ie meet on arrival at HKT), or

2. check bags through only as far as BKK, clear immigartion in BKK, collect bags in BKK AND then checkin for the domestic sector - this way the 2 of you can meet-up 'land-side', check in together, and loaf in the lounge together before th flight.

A bit more hassle - I guess it depends on the depth of the friendship!