If you believe you are playing with a cheater on your fireteam, leave the game and report them immediately.

If you get a short suspension for quitting, just know that it's better than an account ban.

Tell us what you saw. Provide video if possible. https://www.bungie.net/en/Help/Troubleshoot?oid=13967

"But what if I matchmake with a cheater in Freelance or something like that?"

When I say Fireteam, I specifically mean you forming a pre-made fireteam and jumping into an activity.