I agree with scrabulista. If you want the best, most literal translation, go with that. However, if your friend is someone with an appreciation of the tradition of Latin being used in the Western Liturgy, and wants to use Biblical language from the Psalms of the Latin used in the Divine Office of the Catholic Church, a less-literal alternative may be:
Servus Firmamenti Mei
(lit. "The servant of my strength/stay/firmament")
See below for my reasoning.
You will most likely want to go with "Servus Petrae" that scabulista mentioned. However, Psalm 17 (18), is where the famous phrase "The Lord is my rock" comes from. Now, in the Clementine Vulgate, and therefore the version that was in use by the Catholic Church since at least the Council of Trent, the Lord is referred to in this way in Psalm 17 (18) "Dominus firmamentum meum" (lit. "The Lord (is) my strength/support").
Now, most translations of Psalm 17 (18) actually go something more like "The Lord is my rock", so I am led to believe that this is probably what the original Hebrew says.
In any case, if your friend is a Catholic with an appreciation for the Latin of the Catholic Church, they therefore may see this as just as good of a translation. For myself as a Catholic, when I see "Servus petrae", I get either the sense of "Servant of the Rock (e.g. the Lord)" OR "Servant of the rock (e.g. St. Peter/the Pope)".