Embracing spiritual life is a significant stepping stone for many individuals. For some it will not involved adherence to any particular religious group; for others it will be a deepening of their attachment to their own religious tradition or even the conversion to a religion into which they were not born. In Hebrew, the former is called "Baal Tshuva" (Baalat Tshuva for women) and the latter is called "Ger" (Gera for women).
The newly religious or the new convert wants to celebrate his or her new status and all that it signifies. You can celebrate with them by offering a modest yet meaningful present or with a more expensive gift. Here I will make a few suggestions toward the modest end of the spectrum.
Sometimes friends and family share the joy of the Baal Tshuva or Ger, but sometimes they are not at all pleased with the new direction upon which their loved one has embarked. Regardless, in order to maintain good relations it is wise to mark the new path with a small token that shows a degree of acceptance of that which cannot be changed.