You shouldn't ever put anything in an enclosure that you don't know for certain is safe. The consequences could be very serious. You might have gotten lucky this time, but I can't seem to find anything to say lilac flowers are safe, so it's much better to be safe than sorry.
Just because he isn't showing signs now there could be long-term consequences if the plant turns out poisonous or such.
For example, if a Dog eats a grape he isn't going to get sick but if he eats a lot of them, especially over a long period of time, they can cause kidney problems and renal failure.