This sentence is not that easy to understand, even for a native speaker. I have an interpretation, but it depends on the context.

Could you maybe provide some context for your sentence? Where did you read it? What was the speaker talking about before?

Anyway, here’s my interpretation: léger can be used in the phrase “un ton léger“, meaning “a light tone”, or “light-hearted”, when you’re not too serious about something.

The sentence would then mean something like:

My friends are light-hearted [when talking about it], but the situation is no joke.

I assume the text is from a book or a play, because omitting the part about the ton would then be a stylistic figure of speech. It needs the reader to make some effort to understand the sentence, making it more subtle and interesting.

Does that make sense in the context of your sentence?

It is rare to see this kind of sentence with this acception of légers in everyday life. People would usually describe the situation as: Mes amis prennent la situation à la légère, instead of characterizing their friends as légers (that would certainly make it less confusing).

Léger : Qui manque de sérieux, qui fait preuve d’imprudence

French people will say: Mes amis prennent la situation à la légère. This means their friends are not putting unnecessary pressure on themself whereas the situation shoud be taken seriously. Translation: taken lightly.