In Kate Greenaway's Language of Flowers, the entry for Acacia reads: Friendship.
A friendly message with a named source
For a friendship-themed note, acacia offers a reference grounded in a particular flower dictionary. The useful detail is the unqualified entry: a separate yellow-acacia entry carries another message. Treat the two as separate readings rather than assuming every acacia color says the same thing.
A modern card can explain the choice in plain words: the flower recalls friendship for you. That communicates more reliably than expecting the recipient to decode an unfamiliar historical convention.
FAQ
Does acacia always mean friendship?
No. This is Greenaway's assigned meaning, not evidence that every culture or recipient uses it.
Source
Kate Greenaway: Language of Flowers, plant-to-meaning entries