Basically, agar agar is interexchangeably with Knox (unflavored gelatin). Both are gelatin products, agar comes from seaweed, know from fish bones or other animal source.
I would make the buche a week in advance and freeze it, so it is very frozen when delivered. Have your friend keep it frozen, or explain that it would need 24 hours in the refrigerator to be ready to serve.
When I made Josephine’s Hello Kitty 2007, I used super super stabilized whipped cream, and the assembled cake was frozen for 3 weeks. After 36 hours of thawing (a ‘round’ cake takes longer to thaw than a log) nobody knew this was previously frozen. My super super stabilized whipped cream calls for both: corn starch and knox.
My Saturday internship at Satura Cakes, uses whipped cream extensivelly. All completed cakes with whipped cream are frozen, thawed, refrozen, rethawed, really. Whipped cream will freeze and thaw fine, but it will not rewhip/re-spread. So always freeze when at completed/applied stage (fill your cake with it, pipe with it, then freeze). We freeze often more than once due to mass production stage. For example: when making a chocolate ganache layered cupcake (in a cup), we first put a layer of sponge cake, then whipped cream, then a secon layer of sponge cake, then freeze. This is prep stage #1. Then, as needed, we take this frozen prep stage #1 out of the freezer, apply the layers of ganache on top, and the final piping, then refreeze. We deliver these frozen to the retail store, and they thaw the final product as needed. Never, the whipped cream is rewhipped or touched, after frozen.