In my dialogue with Dani Vaucher below I make reference to Wilyam Sulayman Qilada (ed.), Kitāb al-Disqūlīyah: taʿālīm al-rusul (Cairo : Dār al-Jīl lil-Ṭibāʿah, 1979).
My parishioner Mohammed Basith Awan (remember that in the Church of England even Muslims are parishioners… they just have to live in the parish!), a far better Arabist than I, has had a look at it, and has determined that this is the “lost Coptic Didascalia” (again, see posts below) described by Baumstark and found in Codex Borg. Ar. 22. This ms also contains an Arabic version of the Testamentum Domini.
Specialists in this field (among whom I do not count myself) may learn with interest that the Vatican Library has digitized this codex. It may be read here. Coptic marginal annotations clearly indicate an Egyptian provenance.