Probably simply means that Lanfear died, when she was held by the Finn, and when she next awoke in her new body as Cyndane, she had less OP strength. It could also point to a burning out/stilling, when she fell through the doorway, though.

In any case, maybe one can't rule it out completely, but it seems at least very far-fetched that the Finn somehow managed to take just a little part of Lanfear's Power for some reason, but then killed her. It seems more likely that neither Moiraine nor Lanfear got any wishes, but were simply made prisoners.

The other theories to explain Cyndane's strength drop are definitely more convincing and more likely to be true, IMO.