On a website a lady said she uses this at times for her buttercream flowers she makes so the buttercream will harden more. Has anyone heard of this or tried this?

Cream of tartar
1. is used in beaten egg products like meringues to strengthen and stabilize the proteins in egg white for structure.
2. retards re-crystallization in sugar products eg icings and confectionery.
3. inhibits the formation of crystals in the liquids brewed soft drinks.

What sort of buttercream is she using ?

It would have no effect on basic buttercream [fat and sugar]. It may strengthen the egg white, whilst beating, when you are making the Italian or Swiss meringue part of IMBC or SMBC.

I could not see it having any effect on crusting: it specifically functions against that ...