By all means allow them to laugh......laughter is very therapeutic; it's a release of built up (negative) energy which was implanted during a moment of trauma.
In Dianetics my clients will yawn dozens of times, then sometimes have tears roll down their cheeks, and usually at the end of the session will laugh. Sometimes it's a nice belly laugh that lights them up like a Xmas tree, sometimes it's a gush of laughter like a pressure cooker letting out the steam.
Either way, it's wonderful, so never, never, never, never, interfere with the person's laughter, but rather foster it.
Sit back and listen, and allow it to come out.
Then, watch their faces glow with a wonderful smile.
Best of success!