Of course it would have been tested already; Fiona had no doubt about that. But the question she would pose Swallowtail might lead to getting more details about the poison, and how best to treat its effects, or acquiescence to their plans, just as much as it might lead to a stern refusal. Fiona did not forget, however, that Lachlan had waited until Morrigan volunteered to test the poison before he had formulated this objection. She wasn't sure whether he wanted to fool her or himself most, but she let him. For now, it did not matter. Swallowtail would set their course.
Instead, she listened attentively to his answer on the topic of his trust in Morrigan. "All right," she agreed after a few seconds. She trusted him, and so she would give Morrigan the benefit of the doubt. "Then I suggest you have a long talk with her about her questioning our motives. It will not yield any good, either for her or for us."
She was certain Lachlan had already meant to talk to his sister, but she had to make her own stance clear on the topic. The Resistance came first, before anything else. Lachlan knew it as well as she did, and she was so thankful for that. This meeting would have been that much more difficult without his presence by her side.