What is the ruling regarding my laughing when one of them described Fiqh as "bloody and cumbersome," even though I did not intend to mock it?

It is obligatory for a Muslim to guard his tongue. The word spoken by the colleague may have been to express the difficulty of the subject, or to mock matters of Fiqh, which is a grave matter and a great sin. Advice should have been given, and the danger of the word should have been clarified. As for laughter at mockery of religion or otherwise, if it was overwhelming or for another purpose other than رضا (acceptance) of disbelief or the unlawful, then it is not considered disbelief or unlawful. We hope that you will not bear sin for what emanated from you unintentionally, but it is better for a Muslim to show displeasure on his face and be distressed by the ill manners in speaking about matters of Sharia, rather than to laugh.