I say that there should always be some diologue. It is the DM's responsibility to shape the world that the PC grew up in, and if he is high enough to have levels in Lormaster, the character himself would have some idea of what magic would and would not do, especially if he is casting the spell himself. A friendly warning between friends would at least be called for.
As a DM, I would say no, but if you have a mind to give a little, I would have him cast with all the restrictions of a Diviner, but give him only some of the benifits up front. Give him the extra spells up to a level equal to his Intelligence modifier, i.e. a +3 int. would give him the bonus 3rd level and lower divination spells, then every new level he gets, he gets the next level of spells, (4th, ect.). Maybe even assign him the 20% x.p. loss for multi classing. This would represent him giving up whole fields of magic, a difficult and painful thing to do, and represent the hardships of learning everything from scratch. As far as Loremaster goes, or any other prestige class for that matter, he should earn those levels from where he is at.