I believe the problem is that in order to continue to train at both places he may need to:
a. lie (eventually get caught out)
b. upset the southern dojo sensei and deshi
Although perhaps Matthew is right. Be honest, train at both places and the southern dojo people will have to learn to get over it. (or boot you out)
Be a bridge builder !!!
The way I see it, as long as he is treating the southern dojo with due respect and honoring the southern sensei's authority over what goes on in his dojo, he shouldn't have to lie about anything.
Of course, we've all met the douchebag student who likes to interrupt class and say, "My other
sensei does it this
way." If you become that guy, then your honesty is infringing on a sensei's authority in his own dojo. But as long as your honesty is tempered with restraint, humility, and proper respect, no one should have anything to complain about.