The 'trial lesson' is, by accepted definition, a lesson in a course leading to the issue of a PPL and an FI rating is, therefore, required to conduct it. In any case this will all be resolved, in the UK at least, on 1 July next year when Annex 1 to the Aircrew Regulation (a.k.a. Part-FCL) is implemented. Thereafter, nobody may give any flight instruction in an aircraft unless they hold an appropriate instructor certificate.