It may be possible to privately stock the lake with rainbow but a number of permits and licenses are likely required. First the Province's legal position is there is no such thing as a private lake in the Province. The land around the lake may all be held under 'fee simple' title but the lake itself is owned by the Crown. Essentially your friend would need a license and permit to operate a trout farm and permits to transport fish. Permits to transport are required every time fish are brought in or moved out. Next question is can the lake sustain live trout? - summer and winter can be real challenges. A good question is are there any fish living in the lake now or has it sustained fish in the past? However with some effort the lake could be made livable if it is not. Getting Brook Trout may be extremely difficult. I don't think the FFSB sells it's stock though I believe there may still be some commercial rearing of Brook trout in the US.
Lots of challenges and possibly not worth the bother or the expense.
There are no fish now, but there were in the past. The lake winterkills often, but as I said, the owner is willing to install aerators to deal with that issue. We are aware of the challenges, that is why I have posted about it here, to pick some brains.
The biologist in charge will look into the carp issue, because he himself likes the idea of having grass eaters co-habitating with insect/fish eaters. Unfortunately, his hands are tied as far as the stocking part goes.
This lake lends itself to experimentation, and the new owner has the willingness and money to make it work. I'm supposed to help him navigate the red tape.